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/en/errors.json

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