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/errors.json

@@ -0,0 +1,23 @@
+{
+  "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"
+  }
+}