Add auth expiry interceptor and session-expired redirect

frontend/src/providers/AuthProvider.tsx

@@ -10,10 +10,13 @@
 import {
   createContext,
   useCallback,
+  useEffect,
   useMemo,
   useState,
 } from "react";
+import { useNavigate } from "react-router-dom";
 import * as authApi from "../api/auth";
+import { SESSION_EXPIRED_EVENT } from "../api/client";
 
 // ---------------------------------------------------------------------------
 // Types
@@ -64,6 +67,21 @@ export function AuthProvider({
     token: sessionStorage.getItem(SESSION_KEY),
     expiresAt: sessionStorage.getItem(SESSION_EXPIRES_KEY),
   }));
+  const navigate = useNavigate();
+
+  const handleSessionExpired = useCallback((): void => {
+    sessionStorage.removeItem(SESSION_KEY);
+    sessionStorage.removeItem(SESSION_EXPIRES_KEY);
+    setAuth({ token: null, expiresAt: null });
+    navigate("/login", { replace: true });
+  }, [navigate]);
+
+  useEffect((): (() => void) => {
+    window.addEventListener(SESSION_EXPIRED_EVENT, handleSessionExpired);
+    return (): void => {
+      window.removeEventListener(SESSION_EXPIRED_EVENT, handleSessionExpired);
+    };
+  }, [handleSessionExpired]);
 
   const isAuthenticated = useMemo<boolean>(() => {
     if (!auth.token || !auth.expiresAt) return false;