feat: AI photo identification from product form

When creating a new product (manual entry), a '📷 Scatta foto e identifica con AI'
button appears at the top of the form. Tapping it:
1. Opens a camera modal (same pattern as expiry scanner)
2. User takes photo of product/label
3. Sends to gemini_identify — returns name, brand, category + OpenFoodFacts matches
4. User can pick a specific OFF match (fills barcode + full details via lookup_barcode)
   or tap 'Usa dati AI' to fill just name/brand/category from Gemini
5. All matching fields are auto-filled: name, brand, category, barcode, image, unit/qty
6. Button hidden when editing an existing product (not needed)

index.html

@@ -306,6 +306,12 @@
         </div>
         <form class="form" onsubmit="submitProduct(event)">
             <input type="hidden" id="pf-id">
+            <div id="pf-ai-fill-row" class="form-group">
+                <button type="button" class="btn btn-accent full-width" onclick="captureForAIFormFill()">
+                    📷 Scatta foto e identifica con AI
+                </button>
+                <p class="form-hint" style="text-align:center;margin-top:4px">L'AI compilerà automaticamente i campi del prodotto</p>
+            </div>
             <div class="form-group">
                 <label>🏷️ Nome Prodotto *</label>
                 <input type="text" id="pf-name" class="form-input" required placeholder="Es: Latte intero, Pasta penne rigate..."