test: aggiungi suite completa unit, integration ed e2e

- Unit (12+9): conversion.js (rawToCooked/cookedToRaw, edge case, inversa)
  e storage.js (save/load, round-trip, default fallback)
- Integration (17+12+14): Converter (ricerca, selezione, calcolo, swap, reset),
  MealPlanner (rendering, add/remove, generateShopping, deduplicazione),
  ShoppingList (add, toggle, remove, clearAll, contatore)
- E2E Playwright (6+6+7+10): navigation, meal-planner, converter, shopping-list
- Configurazione: vitest.config.js + playwright.config.js + tests/setup.js
- Script: test, test:coverage, test:e2e, test:e2e:ui

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

tests/setup.js

@@ -0,0 +1,5 @@
+import { beforeEach, afterEach } from 'vitest'
+
+// Svuota localStorage prima e dopo ogni test per garantire isolamento
+beforeEach(() => localStorage.clear())
+afterEach(() => localStorage.clear())