fix(frontend): remove invalid json parsing on shift signup

- Backend `/signup` endpoint returns 200 OK with an empty body (`TypedResults.Ok()`), causing `res.json()` to throw 'Unexpected end of JSON input'. Removed the `res.json()` return.
- Added Suspense boundary in login page to fix `useSearchParams` build error.

frontend/src/app/login/page.tsx

@@ -1,12 +1,12 @@
 'use client';
 
-import { useEffect } from 'react';
+import { useEffect, Suspense } from 'react';
 import { signIn, signOut, useSession } from 'next-auth/react';
 import { useRouter, useSearchParams } from 'next/navigation';
 import { Card, CardHeader, CardTitle, CardContent, CardFooter } from '@/components/ui/card';
 import { Button } from '@/components/ui/button';
 
-export default function LoginPage() {
+function LoginContent() {
   const { status } = useSession();
   const router = useRouter();
   const searchParams = useSearchParams();
@@ -30,7 +30,6 @@ export default function LoginPage() {
   };
 
   return (
-    <div className="flex items-center justify-center min-h-screen bg-gray-50">
     <Card className="w-96">
       <CardHeader>
         <CardTitle className="text-2xl text-center">WorkClub Manager</CardTitle>
@@ -51,6 +50,15 @@ export default function LoginPage() {
         </CardFooter>
       )}
     </Card>
+  );
+}
+
+export default function LoginPage() {
+  return (
+    <div className="flex items-center justify-center min-h-screen bg-gray-50">
+      <Suspense fallback={<Card className="w-96 p-6 text-center">Loading...</Card>}>
+        <LoginContent />
+      </Suspense>
     </div>
   );
 }

frontend/src/hooks/useShifts.ts

@@ -111,7 +111,6 @@ export function useSignUpShift() {
         method: 'POST',
       });
       if (!res.ok) throw new Error('Failed to sign up');
-      return res.json();
     },
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ['shifts', activeClubId] });