feat: dynamic units table with localized names via GET /units

- Add units + unit_translations tables with FK constraints on products and ingredient_mappings
- Normalize products.unit from Russian strings (г, кг) to English codes (g, kg)
- Load units at startup (in-memory registry) and serve via GET /units (language-aware)
- Replace hardcoded _units lists and _mapUnit() functions in Flutter with unitsProvider FutureProvider
- Re-fetches automatically when language changes

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

backend/internal/server/server.go

@@ -11,6 +11,7 @@ import (
 	"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/units"
 	"github.com/food-ai/backend/internal/product"
 	"github.com/food-ai/backend/internal/recognition"
 	"github.com/food-ai/backend/internal/recommendation"
@@ -47,6 +48,7 @@ func NewRouter(
 	// Public
 	r.Get("/health", healthCheck(pool))
 	r.Get("/languages", language.List)
+	r.Get("/units", units.List)
 	r.Route("/auth", func(r chi.Router) {
 		r.Post("/login", authHandler.Login)
 		r.Post("/refresh", authHandler.Refresh)