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/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"
+  }
+}