feat: rename ingredients→products, products→user_products; add barcode/OFF import

- Rename catalog: ingredient/* → product/* (canonical_name, barcode, nutrition per 100g)
- Rename pantry: product/* → userproduct/* (user-owned items with expiry)
- Squash migrations into single 001_initial_schema.sql (clean-db baseline)
- product_categories: add English canonical name column; fix COALESCE in queries
- Remove product_translations: product names are stored in their original language
- Add default_unit_name to product API responses via unit_translations JOIN
- Add cmd/importoff: bulk import from OpenFoodFacts JSONL dump (COPY + ON CONFLICT)
- Diary: support product_id entries alongside dish_id (CHECK num_nonnulls = 1)
- Home: getLoggedCalories joins both recipes and catalog products
- Flutter: rename models/providers/services to match backend rename
- Flutter: add barcode scan flow for diary (mobile_scanner, product_portion_sheet)
- Flutter: localise 6 new keys across 12 languages (barcode scan, portion weight)
- Routes: GET /products/search, GET /products/barcode/{barcode}, /user-products

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

backend/tests/diary/handler_test.go

@@ -140,7 +140,7 @@ func TestCreate_Success(t *testing.T) {
 				Name:      "Oatmeal",
 				Portions:  1,
 				Source:    "manual",
-				DishID:    "dish-1",
+				DishID:    strPtr("dish-1"),
 				CreatedAt: time.Now(),
 			}, nil
 		},
@@ -207,3 +207,5 @@ func TestDelete_Success(t *testing.T) {
 		t.Errorf("expected 204, got %d", recorder.Code)
 	}
 }
+
+func strPtr(s string) *string { return &s }