feat: Implement cross-tab authentication synchronization in AuthProvider

- Add BroadcastChannel API for real-time logout synchronization across tabs
- Implement storage event listener as fallback for older browsers
- When a user logs out in one tab, all other tabs immediately reflect the logout state
- Update tests to verify storage event and BroadcastChannel behavior
- Update Architecture.md to document cross-tab synchronization
- Update Web-Development.md with authentication state management notes

The provider now broadcasts logout messages to other tabs so they immediately
reflect the logout state without requiring a page refresh or additional API calls.
The implementation uses BroadcastChannel as the primary sync mechanism with
storage events as a fallback for older browsers.

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

frontend/src/providers/AuthProvider.tsx

@@ -21,6 +21,13 @@
  * refreshes within the browser tab but is cleared on tab close. The session
  * cookie itself persists according to the backend's cookie settings.
  *
+ * **Cross-Tab Synchronization:**
+ * The provider uses BroadcastChannel API to synchronize logout events across
+ * tabs in real-time. On logout, a message is broadcast to all other tabs so they
+ * immediately reflect the logout state without requiring a page refresh or API call.
+ * If the browser does not support BroadcastChannel, the storage event listener
+ * serves as a fallback (though it only fires when storage is changed in another tab).
+ *
  * **Auth Error Handling:**
  * AuthProvider registers two auth error handlers to ensure that 401/403 errors
  * are never silently swallowed:
@@ -66,6 +73,13 @@ export interface AuthContextValue {
 
 export const AuthContext = createContext<AuthContextValue | null>(null);
 
+// ---------------------------------------------------------------------------
+// Constants
+// ---------------------------------------------------------------------------
+
+const BROADCAST_CHANNEL_NAME = "bangui_auth" as const;
+const BROADCAST_MESSAGE_TYPE_LOGOUT = "logout" as const;
+
 // ---------------------------------------------------------------------------
 // Provider
 // ---------------------------------------------------------------------------
@@ -79,6 +93,9 @@ export const AuthContext = createContext<AuthContextValue | null>(null);
  * On mount, validates the cached session with the backend. While validation
  * is in progress, a loading indicator is shown. If validation fails with 401,
  * the user is logged out. Network errors do not cause logout.
+ *
+ * Listens for cross-tab logout events via BroadcastChannel (or storage events
+ * as fallback) to immediately reflect logout in other tabs.
  */
 export function AuthProvider({
   children,
@@ -110,6 +127,46 @@ export function AuthProvider({
     [],
   );
 
+  // Handle storage events from other tabs (fallback sync mechanism)
+  useEffect((): (() => void) => {
+    const handleStorageChange = (event: StorageEvent): void => {
+      // Only respond to changes in our authentication key
+      if (event.key === STORAGE_KEY_AUTHENTICATED) {
+        // If the storage was cleared in another tab, logout this tab
+        if (event.newValue === null) {
+          handleSessionExpired();
+        }
+      }
+    };
+
+    window.addEventListener("storage", handleStorageChange);
+    return (): void => {
+      window.removeEventListener("storage", handleStorageChange);
+    };
+  }, [handleSessionExpired]);
+
+  // Handle BroadcastChannel messages for real-time cross-tab sync
+  useEffect((): (() => void) => {
+    let channel: BroadcastChannel | null = null;
+
+    try {
+      channel = new BroadcastChannel(BROADCAST_CHANNEL_NAME);
+      channel.onmessage = (event: MessageEvent): void => {
+        if (event.data?.type === BROADCAST_MESSAGE_TYPE_LOGOUT) {
+          handleSessionExpired();
+        }
+      };
+    } catch {
+      // BroadcastChannel not supported; storage event listener serves as fallback
+    }
+
+    return (): void => {
+      if (channel) {
+        channel.close();
+      }
+    };
+  }, [handleSessionExpired]);
+
   useEffect((): (() => void) => {
     setUnauthorizedHandler((): void => {
       handleSessionExpired();
@@ -146,6 +203,15 @@ export function AuthProvider({
       // Always clear local state even if the API call fails (e.g. expired session).
       sessionStorage.removeItem(STORAGE_KEY_AUTHENTICATED);
       setIsAuthenticated(false);
+
+      // Broadcast logout event to other tabs for immediate sync
+      try {
+        const channel = new BroadcastChannel(BROADCAST_CHANNEL_NAME);
+        channel.postMessage({ type: BROADCAST_MESSAGE_TYPE_LOGOUT });
+        channel.close();
+      } catch {
+        // BroadcastChannel not supported; storage event will fire on other tabs automatically
+      }
     }
   }, []);