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/units/handler.go

@@ -0,0 +1,28 @@
+package units
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/food-ai/backend/internal/locale"
+)
+
+type unitItem struct {
+	Code string `json:"code"`
+	Name string `json:"name"`
+}
+
+// List handles GET /units — returns units with names in the requested language.
+func List(w http.ResponseWriter, r *http.Request) {
+	lang := locale.FromContext(r.Context())
+	items := make([]unitItem, 0, len(Records))
+	for _, u := range Records {
+		name, ok := u.Translations[lang]
+		if !ok {
+			name = u.Code // fallback to English code
+		}
+		items = append(items, unitItem{Code: u.Code, Name: name})
+	}
+	w.Header().Set("Content-Type", "application/json")
+	_ = json.NewEncoder(w).Encode(map[string]any{"units": items})
+}