feat: shopping list pantry hints, barcode multi-API fallback (OPF/beauty/Gemini), README disclaimer

- Shopping list: each item now shows 'Hai già Xg in dispensa' for same-family inventory stock
  - Lazy-loads inventory once per shopping page visit (_getShoppingInventoryCache)
  - Matches by first significant token (same logic as related-stock on action page)
  - Green hint below item badges, dark-mode aware (.shopping-pantry-hint)
- Barcode lookup: added Open Products Facts + Open Beauty Facts as step 3;
  Gemini AI (_barcodeLookupGemini) as final step 4 fallback
- Added stockForName PHP endpoint (stock_for_name action) for future use
- Restored missing function signatures for _offFetchProduct() and saveProduct()
  that were accidentally lost when stockForName was added in a previous session
- Translation: added shopping.pantry_hint in it/en/de

translations/en.json

@@ -213,7 +213,8 @@
     "barcode_acquired": "🔖 Barcode scanned: {code}",
     "scan_barcode": "🔖 Scan Barcode",
     "create_named": "Create {name}",
-    "new_without_barcode": "New product without barcode"
+    "new_without_barcode": "New product without barcode",
+    "stock_in_pantry": "Already in pantry:"
   },
   "action": {
     "title": "What do you want to do?",
@@ -474,7 +475,8 @@
     "priority_medium": "Medium",
     "priority_low": "Low",
     "smart_last_update": "Updated {time}",
-    "names_already_updated": "All names are already up to date"
+    "names_already_updated": "All names are already up to date",
+    "pantry_hint": "Already at home: {qty}"
   },
   "ai": {
     "title": "🤖 AI Identification",