Fix GitHub OAuth authentication flow with proper callback handler

app/routes/_public.auth.callback.tsx

@@ -0,0 +1,36 @@
+// @ts-nocheck
+import { json, redirect, type LoaderFunctionArgs } from '@remix-run/cloudflare';
+import { createUserSession, exchangeCodeForToken, fetchGitHubUser, verifyState } from '~/lib/auth/github-oauth.server';
+
+export async function loader({ request }: LoaderFunctionArgs) {
+  const url = new URL(request.url);
+  const code = url.searchParams.get('code');
+  const state = url.searchParams.get('state');
+
+  // Check if code and state are present
+  if (!code || !state) {
+    return json({ error: 'Missing required OAuth parameters' }, { status: 400 });
+  }
+
+  try {
+    // Verify state parameter to prevent CSRF attacks
+    const cookie = await verifyState(request, state);
+
+    // Exchange code for access token
+    const { accessToken } = await exchangeCodeForToken(code, state);
+
+    // Fetch user data from GitHub API
+    const user = await fetchGitHubUser(accessToken);
+
+    // Create user session and redirect to home
+    return createUserSession(
+      request,
+      { user, accessToken },
+      '/', // Redirect to home page after successful login
+    );
+  } catch (error) {
+    console.error('Authentication error:', error);
+
+    return redirect('/login?error=authentication_failed');
+  }
+}