Refactor authentication logic and API client

- Update AuthProvider with improved error handling and token management
- Enhance API client with better request/response handling
- Add comprehensive test coverage for auth flows
- Update documentation with current tasks

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

frontend/src/providers/AuthProvider.tsx

@@ -25,7 +25,7 @@ import {
 } from "react";
 import { useNavigate } from "react-router-dom";
 import * as authApi from "../api/auth";
-import { SESSION_EXPIRED_EVENT } from "../api/client";
+import { setUnauthorizedHandler } from "../api/client";
 
 // ---------------------------------------------------------------------------
 // Context
@@ -75,9 +75,11 @@ export function AuthProvider({
   }, [navigate]);
 
   useEffect((): (() => void) => {
-    window.addEventListener(SESSION_EXPIRED_EVENT, handleSessionExpired);
+    setUnauthorizedHandler((): void => {
+      handleSessionExpired();
+    });
     return (): void => {
-      window.removeEventListener(SESSION_EXPIRED_EVENT, handleSessionExpired);
+      setUnauthorizedHandler(null);
     };
   }, [handleSessionExpired]);