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>

backend/internal/server/server.go

@@ -8,6 +8,7 @@ import (
 	"github.com/food-ai/backend/internal/diary"
 	"github.com/food-ai/backend/internal/home"
 	"github.com/food-ai/backend/internal/ingredient"
+	"github.com/food-ai/backend/internal/language"
 	"github.com/food-ai/backend/internal/menu"
 	"github.com/food-ai/backend/internal/middleware"
 	"github.com/food-ai/backend/internal/product"
@@ -45,6 +46,7 @@ func NewRouter(
 
 	// Public
 	r.Get("/health", healthCheck(pool))
+	r.Get("/languages", language.List)
 	r.Route("/auth", func(r chi.Router) {
 		r.Post("/login", authHandler.Login)
 		r.Post("/refresh", authHandler.Refresh)