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/classes.json

@@ -0,0 +1,55 @@
+{
+  "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"
+  }
+}