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/registry.go

@@ -0,0 +1,73 @@
+package units
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// Record is a unit loaded from DB with all its translations.
+type Record struct {
+	Code         string
+	SortOrder    int
+	Translations map[string]string // lang → localized name
+}
+
+// Records is the ordered list of active units, populated by LoadFromDB at startup.
+var Records []Record
+
+// LoadFromDB queries units + unit_translations and populates Records.
+func LoadFromDB(ctx context.Context, pool *pgxpool.Pool) error {
+	rows, err := pool.Query(ctx, `
+		SELECT u.code, u.sort_order, ut.lang, ut.name
+		FROM units u
+		LEFT JOIN unit_translations ut ON ut.unit_code = u.code
+		ORDER BY u.sort_order, ut.lang`)
+	if err != nil {
+		return fmt.Errorf("load units from db: %w", err)
+	}
+	defer rows.Close()
+
+	byCode := map[string]*Record{}
+	var order []string
+	for rows.Next() {
+		var code string
+		var sortOrder int
+		var lang, name *string
+		if err := rows.Scan(&code, &sortOrder, &lang, &name); err != nil {
+			return err
+		}
+		if _, ok := byCode[code]; !ok {
+			byCode[code] = &Record{Code: code, SortOrder: sortOrder, Translations: map[string]string{}}
+			order = append(order, code)
+		}
+		if lang != nil && name != nil {
+			byCode[code].Translations[*lang] = *name
+		}
+	}
+	if err := rows.Err(); err != nil {
+		return err
+	}
+
+	result := make([]Record, 0, len(order))
+	for _, code := range order {
+		result = append(result, *byCode[code])
+	}
+	Records = result
+	return nil
+}
+
+// NameFor returns the localized name for a unit code.
+// Falls back to the code itself when no translation exists.
+func NameFor(code, lang string) string {
+	for _, u := range Records {
+		if u.Code == code {
+			if name, ok := u.Translations[lang]; ok {
+				return name
+			}
+			return u.Code
+		}
+	}
+	return code
+}