feat(frontend): add ignoreCancellation option for background tasks

Allow useNavigationAbortSignal to opt out of navigation-based abort
for long-lived background tasks like polling. Set ignoreCancellation: true
to keep requests alive across route changes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/providers/NavigationCancellationContext.ts

@@ -2,22 +2,36 @@
  * Navigation-aware request cancellation context.
  *
  * Provides a global cancellation mechanism tied to route transitions.
- * When the user navigates to a new route, all AbortSignals obtained from
- * this context are automatically aborted, cancelling in-flight requests
- * associated with the previous route.
+ * When the user navigates to a new route, all AbortSignals whose associated
+ * pathname no longer matches the current route are automatically aborted,
+ * cancelling in-flight requests associated with the previous route.
+ *
+ * Design:
+ * - Each request is associated with the pathname that was active when it started.
+ * - On navigation, all controllers for the old pathname are aborted.
+ * - Requests can opt out of cancellation by setting `ignoreCancellation: true`.
+ * - Background tasks (polling, long-lived syncs) should opt out.
  *
  * Long-lived background fetches (e.g., polling with long TTL) can opt-out
  * by not using this context and instead managing their own lifecycle,
  * or by checking the signal early in their lifecycle.
- *
- * Design notes:
- * - Subscribers are notified immediately when navigation occurs
- * - Multiple consumers can safely subscribe and get independent signals
- * - Signals are generator functions to allow late binding
  */
 
 import { createContext } from "react";
 
+/**
+ * Options for getNavigationSignal().
+ */
+export interface GetNavigationSignalOptions {
+  /**
+   * If true, the signal will not be aborted on navigation.
+   * Use for background tasks that should survive route changes.
+   *
+   * @default false
+   */
+  ignoreCancellation?: boolean;
+}
+
 /**
  * Provides a fresh AbortSignal tied to the current route lifecycle.
  *
@@ -32,16 +46,14 @@ export interface NavigationCancellationContextType {
    * to a different route. This is ideal for route-specific data fetches
    * that should not persist across page transitions.
    *
-   * Example:
-   *   const signal = useNavigationAbortSignal();
-   *   const { items } = useListData({
-   *     fetcher: (sig) => fetchBans(sig || signal),
-   *     // ...
-   *   });
+   * @param options - Optional configuration for the signal
+   * @param options.ignoreCancellation - If true, the signal will not be
+   *   aborted on navigation. Use for background tasks that should survive
+   *   route changes.
    *
    * @returns An AbortSignal that lives for the duration of the current route
    */
-  getNavigationSignal(): AbortSignal;
+  getNavigationSignal(options?: GetNavigationSignalOptions): AbortSignal;
 }
 
 /**