library: refresh scraped cover in grid even when items were rebuilt

refresh_item matched the grid row by object identity, which silently fails
when the library list was rebuilt (background scan/store refresh) while the
scrape dialog was open. Match by release_id + system as a fallback, like
update_item_label already does, so the cover tile updates without a Reload.

app/gui.py

@@ -494,9 +494,17 @@ class CoverShowcase(QWidget):
         pm = QPixmap(str(path))
         if pm.isNull():
             return
+        # Match by identity when possible, falling back to release_id/system
+        # equality (the library may have been rebuilt while the scrape dialog
+        # was open, replacing the item objects).
         for i in range(self.list.count()):
             item = self.list.item(i)
-            if item.data(Qt.UserRole) is it:
+            data = item.data(Qt.UserRole)
+            if data is it or (
+                data is not None
+                and data.get("release_id") == it.get("release_id")
+                and (data.get("system") or "games") == system
+            ):
                 item.setIcon(self._fit_icon(pm))
                 if i in self.missing:
                     self.missing.remove(i)