feat(web): add settings sub-pages and connect to backend APIs

- Add GeneralSettingsPage for language, privacy, and notification settings
- Add FeedbackPage for user feedback submission with image upload
- Connect settings to backend PATCH /users/me/settings API
- Implement language preference sync between frontend and backend
- Update login flow to pass language preference and redirect based on user settings
- Add Astro entry pages for /settings/general and /settings/feedback routes
- Update sidebar navigation: language button links to general settings
- Fix account data card to link to profile page
- Remove "deletion" text from account data description

web/src/lib/auth.ts

@@ -30,6 +30,34 @@ interface SessionResponse {
   user: { id: string; email: string };
 }
 
+// --- Language mapping ---
+
+/**
+ * Map frontend locale to backend BCP-47 language tag
+ */
+export function localeToBackendLanguage(locale: string): string {
+  const mapping: Record<string, string> = {
+    'zh': 'zh-CN',
+    'zh_Hant': 'zh-TW',
+    'en': 'en-US',
+  };
+  return mapping[locale] || 'zh-CN';
+}
+
+/**
+ * Map backend BCP-47 language tag to frontend locale
+ */
+export function backendLanguageToLocale(lang: string): string {
+  const mapping: Record<string, string> = {
+    'zh-CN': 'zh',
+    'zh-TW': 'zh_Hant',
+    'zh-Hant': 'zh_Hant',
+    'en-US': 'en',
+    'en': 'en',
+  };
+  return mapping[lang] || 'zh';
+}
+
 // --- Storage ---
 
 export function getAuth(): AuthData | null {