Fix post-login/setup navigation: use hard redirect instead of goto

goto('/') races with layout's onMount auth check, causing redirect
back to /login. Use window.location.href for full page reload which
re-initializes layout with authenticated state.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/routes/login/+page.svelte

@@ -27,7 +27,7 @@
 		submitting = true;
 		try {
 			await login(username, password);
-			goto('/');
+			window.location.href = '/';
 		} catch (err: any) {
 			error = err.message || 'Login failed';
 		}

frontend/src/routes/setup/+page.svelte

@@ -21,7 +21,7 @@
 		submitting = true;
 		try {
 			await setup(username, password);
-			goto('/');
+			window.location.href = '/';
 		} catch (err: any) { error = err.message || 'Setup failed'; }
 		submitting = false;
 	}