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);
     }
   }

client/lib/features/profile/profile_screen.dart

@@ -123,7 +123,8 @@ class _ProfileBody extends StatelessWidget {
         _InfoCard(children: [
           _InfoRow(
             'Язык',
-            supportedLanguages[user.preferences['language'] as String? ?? 'ru'] ??
+            ref.watch(supportedLanguagesProvider).valueOrNull?[
+                    user.preferences['language'] as String? ?? 'ru'] ??
                 'Русский',
           ),
         ]),
@@ -590,17 +591,23 @@ class _EditProfileSheetState extends ConsumerState<EditProfileSheet> {
                     const SizedBox(height: 20),
 
                     // Language
-                    DropdownButtonFormField<String>(
-                      value: _language,
-                      decoration:
-                          const InputDecoration(labelText: 'Язык интерфейса'),
-                      items: supportedLanguages.entries
-                          .map((e) => DropdownMenuItem(
-                                value: e.key,
-                                child: Text(e.value),
-                              ))
-                          .toList(),
-                      onChanged: (v) => setState(() => _language = v),
+                    ref.watch(supportedLanguagesProvider).when(
+                      data: (languages) => DropdownButtonFormField<String>(
+                        value: _language,
+                        decoration: const InputDecoration(
+                            labelText: 'Язык интерфейса'),
+                        items: languages.entries
+                            .map((e) => DropdownMenuItem(
+                                  value: e.key,
+                                  child: Text(e.value),
+                                ))
+                            .toList(),
+                        onChanged: (v) => setState(() => _language = v),
+                      ),
+                      loading: () => const Center(
+                          child: CircularProgressIndicator()),
+                      error: (_, __) =>
+                          const Text('Не удалось загрузить языки'),
                     ),
                     const SizedBox(height: 32),