feat: introduce i18n system and class invitation codes

Add complete i18n infrastructure using next-intl (cookie-driven, without i18n routing) with zh-CN/en dictionary files, locale switcher, and NextIntlClientProvider in root layout. Add class invitation code system with new class_invitation_codes table, data-access layer (generate/validate/consume/revoke), server actions with permission checks, rate limiting, and audit logging. Add class-invitation-manager UI component. Refactor onboarding stepper to use i18n translations and accept new invitation code format (6-char alphanumeric) with backward compatibility for legacy 6-digit codes.

src/shared/i18n/locale.ts

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+/**
+ * 项目 i18n 配置（without i18n routing 模式）。
+ *
+ * 设计决策（v3 引入完整 i18n 体系）：
+ * - 采用 next-intl 4.x，官方推荐的 Next.js App Router 方案
+ * - 不使用 `[locale]` 路由段，避免破坏现有 (auth)/(dashboard)/(onboarding) 路由组结构
+ * - locale 通过 cookie 持久化，SSR 时从 cookie 读取
+ * - 字典放在 shared/i18n/messages/，符合三层架构约束
+ *
+ * 支持的 locale：
+ * - zh-CN（默认）：简体中文
+ * - en：英文
+ */
+
+export const LOCALES = ["zh-CN", "en"] as const;
+export type Locale = (typeof LOCALES)[number];
+
+export const DEFAULT_LOCALE: Locale = "zh-CN";
+
+/** Cookie 名称，与 proxy.ts / request.ts 保持一致 */
+export const LOCALE_COOKIE = "NEXT_LOCALE";
+
+/** Cookie 属性：1 年有效期，全站可读 */
+export const LOCALE_COOKIE_OPTIONS = {
+  maxAge: 60 * 60 * 24 * 365,
+  path: "/",
+  sameSite: "lax" as const,
+};
+
+/**
+ * 校验字符串是否为受支持的 locale。
+ * 用于从 cookie/header 读取后的容错处理。
+ */
+export function isLocale(value: string | null | undefined): value is Locale {
+  return typeof value === "string" && (LOCALES as readonly string[]).includes(value);
+}
+
+/**
+ * 将任意字符串归一化为合法 locale，非法值回退到默认。
+ */
+export function normalizeLocale(value: string | null | undefined): Locale {
+  return isLocale(value) ? value : DEFAULT_LOCALE;
+}

src/shared/i18n/messages/en/auth.json

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+{
+  "login": {
+    "title": "Sign In",
+    "subtitle": "Sign in to your Next_Edu account",
+    "email": "Email",
+    "password": "Password",
+    "rememberMe": "Remember me",
+    "signIn": "Sign In",
+    "signingIn": "Signing in...",
+    "forgotPassword": "Forgot password?",
+    "noAccount": "Don't have an account?",
+    "register": "Register"
+  },
+  "register": {
+    "title": "Register",
+    "subtitle": "Create your Next_Edu account",
+    "email": "Email",
+    "password": "Password",
+    "confirmPassword": "Confirm password",
+    "name": "Name",
+    "signUp": "Sign Up",
+    "signingUp": "Signing up...",
+    "hasAccount": "Already have an account?",
+    "signIn": "Sign In"
+  },
+  "errors": {
+    "invalidCredentials": "Invalid email or password",
+    "emailExists": "This email is already registered",
+    "passwordTooWeak": "Password is too weak"
+  }
+}

src/shared/i18n/messages/en/classes.json

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+{
+  "invitation": {
+    "title": "Class Invitation Code",
+    "generate": "Generate Code",
+    "regenerate": "Regenerate",
+    "revoke": "Revoke",
+    "copy": "Copy",
+    "copied": "Copied",
+    "code": "Code",
+    "status": "Status",
+    "expiresAt": "Expires At",
+    "usedCount": "Used Count",
+    "maxUses": "Max Uses",
+    "createdAt": "Created At",
+    "note": "Note",
+    "neverExpires": "Never expires",
+    "unlimited": "Unlimited",
+    "active": "Active",
+    "disabled": "Disabled",
+    "expired": "Expired",
+    "exhausted": "Exhausted",
+    "list": "Invitation Codes",
+    "empty": "No invitation codes",
+    "join": "Join Class",
+    "joinPlaceholder": "Enter 6-character code",
+    "joinSuccess": "Joined class successfully",
+    "joinFailed": "Failed to join class",
+    "invalidCode": "Invalid or expired invitation code",
+    "rateLimited": "Too many attempts, please try again later",
+    "alreadyInClass": "You are already in this class",
+    "subjectConflict": "This subject already has a teacher for this class",
+    "generateSuccess": "Invitation code generated",
+    "generateFailed": "Failed to generate invitation code",
+    "revokeSuccess": "Invitation code revoked",
+    "revokeFailed": "Failed to revoke invitation code",
+    "regenerateSuccess": "Invitation code regenerated",
+    "regenerateConfirm": "The old code will be invalidated immediately after regeneration. Continue?",
+    "revokeConfirm": "The code will be invalidated immediately after revocation. Continue?",
+    "expiresInHours": "Validity (hours)",
+    "maxUsesLabel": "Max uses",
+    "customNote": "Note (optional)",
+    "customNotePlaceholder": "e.g., Back-to-school temporary code",
+    "generateWithCustom": "Generate custom code",
+    "defaultDuration": "Never expires",
+    "defaultMaxUses": "Unlimited"
+  },
+  "class": {
+    "title": "Class",
+    "name": "Class Name",
+    "grade": "Grade",
+    "homeroomTeacher": "Homeroom Teacher",
+    "studentCount": "Student Count",
+    "actions": "Actions"
+  }
+}

src/shared/i18n/messages/en/common.json

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+{
+  "app": {
+    "title": "Next_Edu - K12 Smart Education System",
+    "description": "Enterprise Grade K12 Education Management System"
+  },
+  "actions": {
+    "save": "Save",
+    "cancel": "Cancel",
+    "delete": "Delete",
+    "confirm": "Confirm",
+    "search": "Search",
+    "reset": "Reset",
+    "submit": "Submit",
+    "back": "Back",
+    "next": "Next",
+    "previous": "Previous",
+    "finish": "Finish",
+    "skip": "Skip",
+    "retry": "Retry",
+    "close": "Close",
+    "add": "Add",
+    "remove": "Remove",
+    "edit": "Edit",
+    "view": "View",
+    "loading": "Loading...",
+    "saving": "Saving...",
+    "submitting": "Submitting..."
+  },
+  "status": {
+    "success": "Success",
+    "failure": "Failure",
+    "pending": "Pending",
+    "active": "Active",
+    "disabled": "Disabled",
+    "expired": "Expired",
+    "exhausted": "Exhausted"
+  },
+  "locale": {
+    "switch": "Switch language",
+    "zh-CN": "中文",
+    "en": "English"
+  },
+  "empty": {
+    "default": "No data",
+    "noResults": "No matching results found"
+  },
+  "pagination": {
+    "previous": "Previous",
+    "next": "Next",
+    "page": "Page {page}",
+    "total": "{total} items"
+  }
+}

src/shared/i18n/messages/en/errors.json

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+{
+  "auth": {
+    "required": "Please sign in first",
+    "forbidden": "Access denied",
+    "sessionExpired": "Session expired, please sign in again"
+  },
+  "network": {
+    "requestFailed": "Request failed, please try again later",
+    "timeout": "Request timed out",
+    "serverError": "Server error"
+  },
+  "validation": {
+    "required": "This field is required",
+    "invalidFormat": "Invalid format",
+    "tooShort": "Too short",
+    "tooLong": "Too long"
+  },
+  "db": {
+    "notFound": "Record not found",
+    "duplicate": "Record already exists",
+    "constraintViolation": "Data constraint violation"
+  }
+}

src/shared/i18n/messages/en/onboarding.json

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+{
+  "title": "First Login Onboarding",
+  "description": "Complete your first login setup",
+  "steps": {
+    "roleConfirm": "Role Confirmation",
+    "basicInfo": "Basic Information",
+    "roleInfo": "Role Information",
+    "complete": "Complete"
+  },
+  "role": {
+    "yourRole": "Your Role",
+    "allRoles": "All roles: {roles}",
+    "adminAssigned": "Roles are pre-assigned by administrators. Contact an administrator to change.",
+    "admin": "Administrator",
+    "teacher": "Teacher",
+    "student": "Student",
+    "parent": "Parent",
+    "grade_head": "Grade Head",
+    "teaching_head": "Teaching Head"
+  },
+  "form": {
+    "name": "Name",
+    "nameRequired": "Please enter your name",
+    "nameMax": "Name cannot exceed 50 characters",
+    "phone": "Phone",
+    "phoneRequired": "Please enter your phone number",
+    "phoneInvalid": "Please enter a valid 11-digit phone number starting with 1",
+    "address": "Address",
+    "addressMax": "Address cannot exceed 200 characters"
+  },
+  "teacher": {
+    "classCodes": "Class Invitation Codes",
+    "classCodesOptional": "Class invitation codes (optional, multiple allowed)",
+    "classCodesPlaceholder": "One per line or comma-separated, 6 alphanumeric characters",
+    "classCodesHint": "The server validates the code and checks whether the subject is already assigned to another teacher.",
+    "subjects": "Teaching Subjects",
+    "subjectsOptional": "Teaching subjects (optional, multiple selection)",
+    "subjectsHint": "Multiple subjects can be selected. The server binds the teacher to each subject for each class. If a subject already has a teacher, the binding will be rejected."
+  },
+  "student": {
+    "classCodesOptional": "Class invitation codes (optional)",
+    "classCodesPlaceholder": "One per line or comma-separated, 6 alphanumeric characters",
+    "classCodesHint": "Skip this if an administrator has pre-assigned your class."
+  },
+  "parent": {
+    "bindHint": "Bind children via child email + child birthday + last 4 digits of child phone (at least one complete entry required, multiple allowed).",
+    "childN": "Child {index}",
+    "childEmail": "Child Email",
+    "childEmailRequired": "Please enter the child's email",
+    "childEmailInvalid": "Invalid child email format",
+    "childBirthDate": "Child Birthday",
+    "childBirthDateRequired": "Please enter the child's birthday",
+    "childBirthDateInvalid": "Invalid child birthday format (YYYY-MM-DD)",
+    "childPhoneSuffix": "Child Phone Last 4 Digits",
+    "childPhoneSuffixRequired": "Please enter the last 4 digits of the child's phone",
+    "childPhoneSuffixInvalid": "Invalid last 4 digits format",
+    "childRelation": "Relationship",
+    "childRelationPlaceholder": "Father / Mother / Other",
+    "addChild": "Add Child"
+  },
+  "complete": {
+    "ready": "Ready to Finish",
+    "readyHint": "Click finish to enter the system."
+  },
+  "validation": {
+    "needNamePhone": "Please enter your name and phone",
+    "needOneChild": "Please complete at least one child's binding information"
+  },
+  "progress": {
+    "label": "Onboarding progress"
+  },
+  "toast": {
+    "completeSuccess": "Setup complete",
+    "partialFailure": "Setup complete, but {count} binding(s) failed",
+    "submitFailed": "Submission failed",
+    "inputInvalid": "Input validation failed"
+  }
+}

src/shared/i18n/messages/zh-CN/auth.json

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+{
+  "login": {
+    "title": "登录",
+    "subtitle": "使用您的账号登录 Next_Edu",
+    "email": "邮箱",
+    "password": "密码",
+    "rememberMe": "记住我",
+    "signIn": "登录",
+    "signingIn": "登录中...",
+    "forgotPassword": "忘记密码？",
+    "noAccount": "还没有账号？",
+    "register": "注册"
+  },
+  "register": {
+    "title": "注册",
+    "subtitle": "创建 Next_Edu 账号",
+    "email": "邮箱",
+    "password": "密码",
+    "confirmPassword": "确认密码",
+    "name": "姓名",
+    "signUp": "注册",
+    "signingUp": "注册中...",
+    "hasAccount": "已有账号？",
+    "signIn": "登录"
+  },
+  "errors": {
+    "invalidCredentials": "邮箱或密码错误",
+    "emailExists": "该邮箱已被注册",
+    "passwordTooWeak": "密码强度不足"
+  }
+}

src/shared/i18n/messages/zh-CN/classes.json

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+{
+  "invitation": {
+    "title": "班级邀请码",
+    "generate": "生成邀请码",
+    "regenerate": "重新生成",
+    "revoke": "撤销",
+    "copy": "复制",
+    "copied": "已复制",
+    "code": "邀请码",
+    "status": "状态",
+    "expiresAt": "过期时间",
+    "usedCount": "已用次数",
+    "maxUses": "最大次数",
+    "createdAt": "创建时间",
+    "note": "备注",
+    "neverExpires": "永久有效",
+    "unlimited": "不限",
+    "active": "有效",
+    "disabled": "已禁用",
+    "expired": "已过期",
+    "exhausted": "已用尽",
+    "list": "邀请码列表",
+    "empty": "暂无邀请码",
+    "join": "加入班级",
+    "joinPlaceholder": "请输入 6 位邀请码",
+    "joinSuccess": "加入班级成功",
+    "joinFailed": "加入班级失败",
+    "invalidCode": "邀请码无效或已失效",
+    "rateLimited": "尝试过于频繁，请稍后再试",
+    "alreadyInClass": "你已在该班级中",
+    "subjectConflict": "该班级此科目已有任课教师",
+    "generateSuccess": "邀请码生成成功",
+    "generateFailed": "邀请码生成失败",
+    "revokeSuccess": "邀请码已撤销",
+    "revokeFailed": "邀请码撤销失败",
+    "regenerateSuccess": "邀请码已重新生成",
+    "regenerateConfirm": "重新生成后旧邀请码将立即失效，确定继续吗？",
+    "revokeConfirm": "撤销后此邀请码将立即失效，确定继续吗？",
+    "expiresInHours": "有效期（小时）",
+    "maxUsesLabel": "最大使用次数",
+    "customNote": "备注（可选）",
+    "customNotePlaceholder": "如：开学季临时码",
+    "generateWithCustom": "生成自定义邀请码",
+    "defaultDuration": "永久有效",
+    "defaultMaxUses": "不限次数"
+  },
+  "class": {
+    "title": "班级",
+    "name": "班级名称",
+    "grade": "年级",
+    "homeroomTeacher": "班主任",
+    "studentCount": "学生人数",
+    "actions": "操作"
+  }
+}

src/shared/i18n/messages/zh-CN/common.json

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+{
+  "app": {
+    "title": "Next_Edu - K12 智慧教务系统",
+    "description": "企业级 K12 教务管理系统"
+  },
+  "actions": {
+    "save": "保存",
+    "cancel": "取消",
+    "delete": "删除",
+    "confirm": "确认",
+    "search": "搜索",
+    "reset": "重置",
+    "submit": "提交",
+    "back": "返回",
+    "next": "下一步",
+    "previous": "上一步",
+    "finish": "完成",
+    "skip": "跳过",
+    "retry": "重试",
+    "close": "关闭",
+    "add": "添加",
+    "remove": "移除",
+    "edit": "编辑",
+    "view": "查看",
+    "loading": "加载中...",
+    "saving": "保存中...",
+    "submitting": "提交中..."
+  },
+  "status": {
+    "success": "成功",
+    "failure": "失败",
+    "pending": "待处理",
+    "active": "已启用",
+    "disabled": "已禁用",
+    "expired": "已过期",
+    "exhausted": "已用尽"
+  },
+  "locale": {
+    "switch": "切换语言",
+    "zh-CN": "中文",
+    "en": "English"
+  },
+  "empty": {
+    "default": "暂无数据",
+    "noResults": "未找到匹配结果"
+  },
+  "pagination": {
+    "previous": "上一页",
+    "next": "下一页",
+    "page": "第 {page} 页",
+    "total": "共 {total} 条"
+  }
+}

src/shared/i18n/messages/zh-CN/errors.json

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+{
+  "auth": {
+    "required": "请先登录",
+    "forbidden": "无权限访问",
+    "sessionExpired": "会话已过期，请重新登录"
+  },
+  "network": {
+    "requestFailed": "请求失败，请稍后重试",
+    "timeout": "请求超时",
+    "serverError": "服务器错误"
+  },
+  "validation": {
+    "required": "此字段为必填",
+    "invalidFormat": "格式错误",
+    "tooShort": "长度过短",
+    "tooLong": "长度过长"
+  },
+  "db": {
+    "notFound": "记录不存在",
+    "duplicate": "记录已存在",
+    "constraintViolation": "数据约束冲突"
+  }
+}

src/shared/i18n/messages/zh-CN/onboarding.json

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+{
+  "title": "首次登录引导",
+  "description": "完成首次登录信息配置",
+  "steps": {
+    "roleConfirm": "角色确认",
+    "basicInfo": "基础信息",
+    "roleInfo": "角色信息",
+    "complete": "完成"
+  },
+  "role": {
+    "yourRole": "你的角色",
+    "allRoles": "全部角色：{roles}",
+    "adminAssigned": "角色由管理员预分配，如需变更请联系管理员。",
+    "admin": "管理员",
+    "teacher": "教师",
+    "student": "学生",
+    "parent": "家长",
+    "grade_head": "年级主任",
+    "teaching_head": "教研组长"
+  },
+  "form": {
+    "name": "姓名",
+    "nameRequired": "请填写姓名",
+    "nameMax": "姓名长度不能超过 50 个字符",
+    "phone": "电话",
+    "phoneRequired": "请填写电话",
+    "phoneInvalid": "请输入有效的手机号（11 位，以 1 开头）",
+    "address": "住址",
+    "addressMax": "住址长度不能超过 200 个字符"
+  },
+  "teacher": {
+    "classCodes": "班级邀请码",
+    "classCodesOptional": "班级邀请码（可选，可多个）",
+    "classCodesPlaceholder": "每行一个或用逗号分隔，6 位字母数字",
+    "classCodesHint": "服务端会校验邀请码有效性及科目是否已被其他教师占用。",
+    "subjects": "教学科目",
+    "subjectsOptional": "教学科目（可选，可多选）",
+    "subjectsHint": "可选多个科目，服务端会为每个班级的每个所选科目绑定任课教师。若班级该科目已有任课教师，绑定将被拒绝。"
+  },
+  "student": {
+    "classCodesOptional": "班级邀请码（可选）",
+    "classCodesPlaceholder": "每行一个或用逗号分隔，6 位字母数字",
+    "classCodesHint": "若管理员已预分配班级，可跳过此项。"
+  },
+  "parent": {
+    "bindHint": "通过子女邮箱 + 子女生日 + 子女手机号后 4 位绑定子女（至少完整填写一个，可添加多个）。",
+    "childN": "子女 {index}",
+    "childEmail": "子女邮箱",
+    "childEmailRequired": "请填写子女邮箱",
+    "childEmailInvalid": "子女邮箱格式错误",
+    "childBirthDate": "子女生日",
+    "childBirthDateRequired": "请填写子女生日",
+    "childBirthDateInvalid": "子女生日格式错误（YYYY-MM-DD）",
+    "childPhoneSuffix": "子女手机号后 4 位",
+    "childPhoneSuffixRequired": "请填写子女手机号后 4 位",
+    "childPhoneSuffixInvalid": "子女手机号后 4 位格式错误",
+    "childRelation": "关系",
+    "childRelationPlaceholder": "父亲 / 母亲 / 其他",
+    "addChild": "添加子女"
+  },
+  "complete": {
+    "ready": "已准备完成",
+    "readyHint": "点击完成后进入系统。"
+  },
+  "validation": {
+    "needNamePhone": "请填写姓名与电话",
+    "needOneChild": "请至少完整填写一个子女的绑定信息"
+  },
+  "progress": {
+    "label": "Onboarding 进度"
+  },
+  "toast": {
+    "completeSuccess": "配置完成",
+    "partialFailure": "配置完成，但 {count} 项绑定失败",
+    "submitFailed": "提交失败",
+    "inputInvalid": "输入校验失败"
+  }
+}