Consolidate auth routes under /auth and rename /u to /user

Moves the login screen from /login to /auth, logout from /logout to
/auth/logout (alongside the existing /auth/login and /auth/callback
endpoints), and the public profile route from /u/[id] to /user/[id].

AUTH_REDIRECT_URI and AUTH_POST_LOGOUT_URI already point at
/auth/callback and / respectively, so no Zitadel config change is
needed for the auth move.

web-player/src/routes/auth/+page.server.ts

@@ -0,0 +1,9 @@
+import { redirect } from '@sveltejs/kit';
+import type { PageServerLoad } from './$types';
+
+export const load: PageServerLoad = ({ locals, url }) => {
+	if (locals.user) {
+		redirect(302, url.searchParams.get('returnTo') ?? '/');
+	}
+	return { returnTo: url.searchParams.get('returnTo') ?? '/' };
+};