fix(login): post local credentials to /auth/callback/local not /signin/local

The Credentials provider in Auth.js processes credentials at the
/callback/:provider endpoint, not /signin/:provider. Posting to
/signin/local hits a code path that just redirects back to the Auth.js
built-in sign-in page. Changing to /callback/local routes the form
submission to the correct handler.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/routes/login/+page.server.ts

@@ -14,7 +14,7 @@ export const load: PageServerLoad = async ({ url }) => {
 
 	return {
 		signinUrl: `${authUrl}/signin/synology`,
-		localSigninUrl: `${authUrl}/signin/local`,
+		localSigninUrl: `${authUrl}/callback/local`,
 		callbackUrl: appRoot,
 		localAuthEnabled,
 		error: resolveErrorMessage(url.searchParams.get('error'))