Fix overlay blocking nav, add header scan button, fix qty defaults, improve expiry UI

- Toast: add pointer-events:none when hidden to prevent invisible overlay blocking bottom nav
- Header: add prominent camera/scan button (📷) in top-right corner with pulse animation
- Product defaults: auto-fix products saved with 'pz/1' that have weight info in notes (re-detects unit/qty from Peso field and updates DB)
- Expiry sections: show relative days ('3 giorni', 'Domani', 'Scaduto da 5 giorni') instead of absolute dates
- Inventory list + dashboard items: use relative expiry labels
- New CSS: alert-item cards with badges, expiring/expired color-coded badges
- Added daysUntilExpiry() utility function

index.html

@@ -19,6 +19,9 @@
 <header class="app-header">
     <div class="header-content">
         <h1 class="header-title" onclick="showPage('dashboard')">🏠 Dispensa</h1>
+        <button class="header-scan-btn" onclick="showPage('scan')" title="Scansiona prodotto">
+            📷
+        </button>
     </div>
 </header>