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/hooks/useNavigationAbortSignal.ts

@@ -0,0 +1,53 @@
+/**
+ * Hook to subscribe to navigation-aware request cancellation.
+ *
+ * Returns an AbortSignal that is automatically aborted when the user
+ * navigates to a different route. Use this signal to cancel API requests
+ * that are specific to the current route and should not survive a navigation.
+ *
+ * Usage:
+ *   const signal = useNavigationAbortSignal();
+ *   const { items } = useListData({
+ *     fetcher: (sig) => fetchBans(sig || signal),
+ *     // ...
+ *   });
+ *
+ * When to use:
+ * - For page-level data fetches that should not persist across navigation
+ * - For user-initiated refetches on the current page
+ * - For paginated lists, search results, filters
+ *
+ * When NOT to use:
+ * - For long-lived background polls (use your own AbortController instead)
+ * - For service-level state syncs (e.g., session validation)
+ * - For actions that may take longer than a user interaction timeout
+ *
+ * Note: The signal may already be aborted at the time you check it,
+ * depending on timing. This is safe — fetchers should handle aborted
+ * signals gracefully by throwing/catching AbortError.
+ */
+
+import { useContext } from "react";
+import { NavigationCancellationContext } from "../providers/NavigationCancellationContext";
+
+/**
+ * Get an AbortSignal for the current route's request lifecycle.
+ *
+ * The returned signal will be aborted when the user navigates away.
+ * All requests using this signal will be automatically cancelled.
+ *
+ * @returns AbortSignal tied to the current route
+ * @throws Error if called outside NavigationCancellationProvider
+ */
+export function useNavigationAbortSignal(): AbortSignal {
+  const context = useContext(NavigationCancellationContext);
+
+  if (!context) {
+    throw new Error(
+      "useNavigationAbortSignal must be used within NavigationCancellationProvider. " +
+        "Wrap your router with <NavigationCancellationProvider> in App.tsx.",
+    );
+  }
+
+  return context.getNavigationSignal();
+}