Cancel in-flight paginated fetches to prevent stale-write races

Rapidly switching filter chips, sort columns, or pagination on the devices
and notifications lists could overlap fetches, and an older response
landing after a newer one would silently overwrite the list with results
that no longer match the active query.

Thread an optional CancelToken into listDevices and listNotifications.
Each State now keeps one active token, cancels it on every new fetch and
on dispose, and bails from both success and catch branches when its token
has been superseded. DioExceptionType.cancel is silenced so internal
cancellations don't surface as user-visible errors. The notifications
list's prior `if (_isLoading) return;` guard is removed so the 1-min
periodic refresh is no longer skipped during user activity.

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

frontend/lib/utils/oott_api.dart

@@ -142,6 +142,7 @@ class BackendAPI {
     bool? sortAscending,
     int? offset,
     int? limit,
+    CancelToken? cancelToken,
   }) async {
     debugPrint('About to call /devices');
 
@@ -162,7 +163,11 @@ class BackendAPI {
       params['page_limit'] = limit ?? _pageSize;
     }
 
-    final response = await _dio.get('/devices', queryParameters: params);
+    final response = await _dio.get(
+      '/devices',
+      queryParameters: params,
+      cancelToken: cancelToken,
+    );
 
     debugPrint('Received: ${response.data}');
 
@@ -257,6 +262,7 @@ class BackendAPI {
     bool? isNew,
     int offset, {
     int? limit,
+    CancelToken? cancelToken,
   }) async {
     debugPrint('About to call /notifications');
 
@@ -268,6 +274,7 @@ class BackendAPI {
         'page_offset': offset,
         'page_limit': limit ?? _pageSize,
       },
+      cancelToken: cancelToken,
     );
 
     debugPrint('Received: ${response.data}');