feat: Implement global request lifecycle cancellation on route transitions

Adds a navigation-aware request cancellation mechanism that automatically
aborts all route-specific API requests when the user navigates to a
different route. This prevents silent state-update errors from responses
arriving after component unmount and conserves bandwidth by cancelling
now-irrelevant requests.

Key additions:
- NavigationCancellationContext: Context for managing route-specific signals
- NavigationCancellationProvider: Provider that detects route changes and
  aborts all signals from the previous route
- useNavigationAbortSignal hook: Allows components to subscribe to
  navigation-aware cancellation signals
- Comprehensive tests for the cancellation lifecycle
- Documentation in Web-Development.md for request lifecycle policy

The provider is placed in the app hierarchy between BrowserRouter and
AuthProvider, ensuring consistent cancellation behavior across all routes.

Long-lived background tasks (polling, session validation) can opt-out by
managing their own AbortController lifecycle.

Closes #23 from Tasks.md: No global cancellation policy on route transitions

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

frontend/src/providers/NavigationCancellationContext.ts

@@ -0,0 +1,56 @@
+/**
+ * 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.
+ *
+ * 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";
+
+/**
+ * Provides a fresh AbortSignal tied to the current route lifecycle.
+ *
+ * Each call returns a new AbortSignal. When the user navigates,
+ * all previously-returned signals are aborted.
+ */
+export interface NavigationCancellationContextType {
+  /**
+   * Get an AbortSignal for the current route's request lifecycle.
+   *
+   * The signal will be aborted automatically when the user navigates
+   * 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),
+   *     // ...
+   *   });
+   *
+   * @returns An AbortSignal that lives for the duration of the current route
+   */
+  getNavigationSignal(): AbortSignal;
+}
+
+/**
+ * React context for navigation-aware cancellation.
+ *
+ * Wrap the application with `NavigationCancellationProvider` to enable
+ * automatic request cancellation on route transitions.
+ */
+export const NavigationCancellationContext =
+  createContext<NavigationCancellationContextType | null>(null);
+
+NavigationCancellationContext.displayName = "NavigationCancellation";