Add "go to last page" and page count to notifications and devices lists

The list endpoints now return a total count alongside the page so the
front-end can show how many pages exist and offer a last-page jump.

Backend: add count(is_new) and count_devices(...) (sharing a WHERE-builder
with list_devices so page and count can't drift), wrap both list responses
in {items, total_count} structs, and register them with utoipa.

Front-end: parse the wrapper shape (dropping the fetch-one-extra trick),
add a Last-page button and a responsive "Page X of Y" / "X / Y" label to
the shared PaginationBar, and track the total in both lists. Notifications
re-sync the count on every fetch and decrement it locally on mark-read/
unread removals so the count stays accurate without a re-fetch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/utils/api/oott_api_notifications.dart

@@ -14,7 +14,7 @@ extension NotificationApi on BackendAPI {
     await _dio.post('/notifications/mark_all_as_old');
   }
 
-  Future<({List<Notification> items, bool hasNextPage})> listNotifications(
+  Future<({List<Notification> items, int totalCount})> listNotifications(
     bool? isNew, {
     int page = 0,
     int perPage = 5,
@@ -25,15 +25,14 @@ extension NotificationApi on BackendAPI {
       queryParameters: {
         'is_new': isNew ?? '',
         'page_offset': page * perPage,
-        'page_limit': perPage + 1,
+        'page_limit': perPage,
       },
       cancelToken: cancelToken,
     );
 
     return _paginate(
-      response.data as List<dynamic>,
+      response.data as Map<String, dynamic>,
       (item) => Notification.fromJson(item),
-      perPage,
     );
   }
 }