Remove auth form reload and keep SPA navigation

frontend/src/app.tsx

@@ -262,6 +262,7 @@ function SetupPage({ onDone }: { onDone: () => Promise<void> }) {
       action="Create admin"
       onSubmit={async (fd) => {
         await api.request('/api/setup', { method: 'POST', body: JSON.stringify(Object.fromEntries(fd)) });
+        nav('/admin');
         await onDone();
       }}
       fields={['username', 'password']}
@@ -276,7 +277,6 @@ function LoginPage({ onDone }: { onDone: () => Promise<void> }) {
       action="Sign in"
       onSubmit={async (fd) => {
         await api.request('/api/login', { method: 'POST', body: JSON.stringify(Object.fromEntries(fd)) });
-        nav('/');
         await onDone();
       }}
       fields={['username', 'password']}
@@ -295,20 +295,28 @@ function AuthCard({
   fields: string[];
   onSubmit: (fd: FormData) => Promise<void>;
 }) {
+  const [submitError, setSubmitError] = useState<string | null>(null);
+
   return (
     <div className="grid min-h-screen place-items-center p-4">
       <form
         className="panel w-full max-w-sm space-y-3"
         onSubmit={async (e) => {
           e.preventDefault();
+          setSubmitError(null);
+          try {
             await onSubmit(new FormData(e.currentTarget));
-          location.reload();
+          } catch (err) {
+            const message = err instanceof Error ? err.message : String(err);
+            setSubmitError(message || 'Request failed. Please try again.');
+          }
         }}
       >
         <h1 className="title-sm">{title}</h1>
         {fields.map((f) => (
           <input key={f} name={f} type={f === 'password' ? 'password' : 'text'} className="input" placeholder={f} required />
         ))}
+        {submitError ? <p className="text-xs text-rose-400">{submitError}</p> : null}
         <button className="btn-subtle w-full" type="submit">{action}</button>
       </form>
     </div>