feat: move supported languages to DB table, expose via GET /languages

- migration 013: create languages table (code PK, native_name, english_name,
  is_active, sort_order) with all 12 existing languages seeded
- locale: add Language struct, Languages []Language, LoadFromDB() — queries
  languages table at startup and populates both Supported map and Languages
  slice; existing Parse/FromContext/FromRequest unchanged
- main.go: call locale.LoadFromDB after pool is ready
- gemini/recipe.go: remove hardcoded langNames map, use locale.Languages
  linear lookup for English name in prompt
- language/handler.go: new package with GET /languages handler returning
  active languages list (no auth required)
- server.go: register GET /languages as public route
- Flutter: add LanguageRepository + languageRepositoryProvider that fetches
  /languages from backend
- language_provider.dart: replace const supportedLanguages map with
  supportedLanguagesProvider (FutureProvider) backed by LanguageRepository
- profile_provider.dart: remove supportedLanguages.containsKey validation —
  backend is source of truth; sync any non-empty language from preferences
- profile_screen.dart: use supportedLanguagesProvider for display name and
  dropdown (async with loading/error states)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/features/profile/profile_provider.dart

@@ -4,6 +4,7 @@ import '../../core/locale/language_provider.dart';
 import '../../shared/models/user.dart';
 import 'profile_service.dart';
 
+
 class ProfileNotifier extends StateNotifier<AsyncValue<User>> {
   final ProfileService _service;
   final void Function(String) _setLanguage;
@@ -33,7 +34,7 @@ class ProfileNotifier extends StateNotifier<AsyncValue<User>> {
   // Propagates the user's preferred language to the global languageProvider.
   void _syncLanguage() {
     final lang = state.valueOrNull?.preferences['language'] as String?;
-    if (lang != null && supportedLanguages.containsKey(lang)) {
+    if (lang != null && lang.isNotEmpty) {
       _setLanguage(lang);
     }
   }