Add AuthProvider tracking Supabase session

frontend/src/auth/AuthProvider.tsx

@@ -0,0 +1,31 @@
+import { createContext, useContext, useEffect, useState, type ReactNode } from 'react';
+import type { Session } from '@supabase/supabase-js';
+import { supabase } from '../lib/supabase';
+
+type AuthContextValue = {
+  session: Session | null;
+  loading: boolean;
+};
+
+const AuthContext = createContext<AuthContextValue>({ session: null, loading: true });
+
+export function AuthProvider({ children }: { children: ReactNode }) {
+  const [session, setSession] = useState<Session | null>(null);
+  const [loading, setLoading] = useState(true);
+
+  useEffect(() => {
+    const { data } = supabase.auth.onAuthStateChange((_event, s) => {
+      setSession(s);
+      setLoading(false);
+    });
+    return () => {
+      data.subscription.unsubscribe();
+    };
+  }, []);
+
+  return <AuthContext.Provider value={{ session, loading }}>{children}</AuthContext.Provider>;
+}
+
+export function useAuth() {
+  return useContext(AuthContext);
+}