fix: stabilize auth-to-tenant flow and correct tenant header mapping

Resolve post-login routing and tenant context issues by proxying frontend API
calls, redirecting authenticated users away from /login, and hardening club
loading with retries/loading guards.

Align tenant identity end-to-end by returning tenantId in /api/clubs/me and
sending X-Tenant-Id from cookie-backed tenantId instead of local clubId,
restoring authorized tasks/shifts data access after club selection.

frontend/next.config.ts

@@ -2,6 +2,15 @@ import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
   output: 'standalone',
+  async rewrites() {
+    const apiUrl = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:5001';
+    return [
+      {
+        source: '/api/:path((?!auth).*)',
+        destination: `${apiUrl}/api/:path*`,
+      },
+    ];
+  },
 };
 
 export default nextConfig;