chore(frontend): add auth guard for session expiration handling

Implemented global 401 response handling to properly redirect users
to login when their session expires:

Changes:

frontend/src/api/client.ts: Added setAuthErrorHandler() callback
pattern and enhanced 401 interceptor to notify auth context
frontend/src/context/AuthContext.tsx: Register auth error handler
that clears state and redirects to /login on 401 responses
tests/core/authentication.spec.ts: Fixed test to clear correct
localStorage key (charon_auth_token)
The implementation uses a callback pattern to avoid circular
dependencies while keeping auth state management centralized.
Auth endpoints (/auth/login, /auth/me) are excluded from the
redirect to prevent loops during initial auth checks.

All 16 authentication E2E tests now pass including:

should redirect to login when session expires
should handle 401 response gracefully
Closes frontend-auth-guard-reload.md

frontend/src/context/AuthContext.tsx

@@ -1,11 +1,28 @@
-import { useState, useEffect, type ReactNode, type FC } from 'react';
-import client, { setAuthToken } from '../api/client';
+import { useState, useEffect, useCallback, type ReactNode, type FC } from 'react';
+import client, { setAuthToken, setAuthErrorHandler } from '../api/client';
 import { AuthContext, User } from './AuthContextValue';
 
 export const AuthProvider: FC<{ children: ReactNode }> = ({ children }) => {
   const [user, setUser] = useState<User | null>(null);
   const [isLoading, setIsLoading] = useState(true);
 
+  // Handle session expiry by clearing auth state and redirecting to login
+  const handleAuthError = useCallback(() => {
+    console.log('Session expired, redirecting to login');
+    localStorage.removeItem('charon_auth_token');
+    setAuthToken(null);
+    setUser(null);
+    // Use window.location for full page redirect to clear any stale state
+    if (window.location.pathname !== '/login') {
+      window.location.href = '/login';
+    }
+  }, []);
+
+  // Register auth error handler on mount
+  useEffect(() => {
+    setAuthErrorHandler(handleAuthError);
+  }, [handleAuthError]);
+
   useEffect(() => {
     const checkAuth = async () => {
       try {