fix: MODIFICA button now edits inventory instance (expiry/location/qty/vacuum)

The MODIFICA button on the action page was opening the catalog editor
(name/brand/category). Users expect it to edit the physical item in hand.

Changes:
- MODIFICA button → openInventoryEdit(): edits the inventory row directly.
  If product is in one location → opens editActionInventoryItem directly.
  If multiple locations → shows a location picker modal first.
- editActionInventoryItem modal already has: qty ±, unit, conf size, location
  buttons, expiry date, vacuum toggle — all fields for the instance.
- Catalog editing (name/brand/category) moved to a small secondary link
  '⚙️ Modifica scheda prodotto' shown discreetly below the action buttons.
- Removed redundant 'Tocca una riga per modificare' hint from status bar.
- Added .btn-link-small CSS class for the secondary catalog-edit link.

assets/css/style.css

@@ -2712,6 +2712,18 @@ body {
     flex-shrink: 0;
 }
 
+.btn-link-small {
+    background: none;
+    border: none;
+    color: var(--text-muted);
+    font-size: 0.8rem;
+    cursor: pointer;
+    padding: 4px 8px;
+    border-radius: 6px;
+    text-decoration: underline;
+    text-underline-offset: 2px;
+}
+
 /* ===== PRODUCT DETAILS CARD (Action Page) ===== */
 .product-details-card {
     background: var(--bg-card);