test: espandi copertura a 110 test e organizza in gruppi

- Aggiunto converter_provider_test.dart (nuovo): stato iniziale, select,
  setGrams, result, swapDirection, reset, search
- Aggiunto qr_test.dart (nuovo): buildQrPayload e parseMealPlanFromQr
  con round-trip e casi di errore
- meal_planner_provider_test: group(), importPlan(), fallback JSON corrotto
- shopping_list_provider_test: group(), addAll con aggregazione quantity,
  fallback JSON corrotto
- shopping_item_test: campi quantity (default, copyWith, JSON round-trip)
- shopping_item_tile_test: group(), visualizzazione (xN)
- conversion_entry_test: caso limite yieldFactor=0
- meal_card_test: group()
- Rimosso widget_test.dart (boilerplate obsoleto)

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

test/features/converter/models/conversion_entry_test.dart

@@ -61,5 +61,14 @@ void main() {
       expect(entry.rawToCooked(0), 0.0);
       expect(entry.cookedToRaw(0), 0.0);
     });
+
+    test('yield = 0: rawToCooked restituisce 0', () {
+      const entry = ConversionEntry(
+        food: 'test',
+        method: 'test',
+        yieldFactor: 0.0,
+      );
+      expect(entry.rawToCooked(100), 0.0);
+    });
   });
 }