feat: add guest mode, bug fixes, and self-hosted config

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/AuthCallback.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useState } from 'react';
+import { useEffect, useRef, useState } from 'react';
 import { useNavigate, useSearchParams } from 'react-router-dom';
 import { useAuth } from '../contexts/AuthContext';
 
@@ -7,8 +7,12 @@ function AuthCallback() {
   const navigate = useNavigate();
   const { login } = useAuth();
   const [error, setError] = useState<string | null>(null);
+  const processedRef = useRef(false);
 
   useEffect(() => {
+    if (processedRef.current) return;
+    processedRef.current = true;
+
     const handleCallback = async () => {
       const token = searchParams.get('token');
       const redirect =