Make notification mark-as-read reliably update the list

Three intertwined fixes so that marking a notification as read from the
trailing menu, swipe-dismiss, and 'Mark all as read' all consistently
update the UI and surface backend failures.

- Guard the mark-read, mark-new, and mark-all calls with try/catch; on
  failure show a snackbar with the mapped error and leave the list
  untouched.
- Use the State's own `mounted` and `context` after the await instead of
  the per-card BuildContext, so the SliverList rebuild reliably runs
  even when the originating NotificationCard's element has been swapped
  out by the reconciliation. _setRead and _markAllAsRead no longer take
  a BuildContext.
- Replace `Dismissible(key: UniqueKey())` with `ValueKey(item.id)` in
  the notification card. UniqueKey was regenerated on every build,
  forcing the Dismissible/PopupMenuButton subtree to be disposed on any
  parent rebuild; the open menu's `mounted` check would then drop
  `onSelected` silently.
- Add `ValueKey(device.macAddress)` to the device list rows for the
  same reason, anchoring each row's Element (and its PopupMenuButton
  State) to its device identity instead of its list position.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/lib/devices/device_list.dart

@@ -286,11 +286,13 @@ class _DeviceListState extends State<DeviceList> with RouteAware {
               final device = _devices[index];
               return isWide
                   ? DeviceRowWide(
+                      key: ValueKey(device.macAddress),
                       device: device,
                       formatter: formatter,
                       onRefresh: () => _fetchPage(_currentPage),
                     )
                   : DeviceRowCompact(
+                      key: ValueKey(device.macAddress),
                       device: device,
                       formatter: formatter,
                       onRefresh: () => _fetchPage(_currentPage),