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/gemini/recipe.go

@@ -5,6 +5,8 @@ import (
 	"encoding/json"
 	"fmt"
 	"strings"
+
+	"github.com/food-ai/backend/internal/locale"
 )
 
 // RecipeGenerator generates recipes using the Gemini AI.
@@ -63,22 +65,6 @@ type NutritionInfo struct {
 	Approximate bool    `json:"approximate"`
 }
 
-// langNames maps ISO 639-1 codes to English language names used in the prompt.
-var langNames = map[string]string{
-	"en": "English",
-	"ru": "Russian",
-	"es": "Spanish",
-	"de": "German",
-	"fr": "French",
-	"it": "Italian",
-	"pt": "Portuguese",
-	"zh": "Chinese (Simplified)",
-	"ja": "Japanese",
-	"ko": "Korean",
-	"ar": "Arabic",
-	"hi": "Hindi",
-}
-
 // goalNames maps internal goal codes to English descriptions used in the prompt.
 var goalNames = map[string]string{
 	"lose":     "weight loss",
@@ -130,9 +116,12 @@ func buildRecipePrompt(req RecipeRequest) string {
 	if lang == "" {
 		lang = "en"
 	}
-	langName, ok := langNames[lang]
-	if !ok {
-		langName = "English"
+	langName := "English"
+	for _, l := range locale.Languages {
+		if l.Code == lang {
+			langName = l.EnglishName
+			break
+		}
 	}
 
 	goal := goalNames[req.UserGoal]