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/core/api/language_repository.dart

@@ -0,0 +1,22 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../auth/auth_provider.dart';
+import 'api_client.dart';
+
+class LanguageRepository {
+  final ApiClient _api;
+  LanguageRepository(this._api);
+
+  Future<Map<String, String>> fetchLanguages() async {
+    final response = await _api.dio.get('/languages');
+    final List<dynamic> items = response.data['languages'] as List;
+    return {
+      for (final item in items)
+        item['code'] as String: item['native_name'] as String,
+    };
+  }
+}
+
+final languageRepositoryProvider = Provider<LanguageRepository>(
+  (ref) => LanguageRepository(ref.watch(apiClientProvider)),
+);