fix(web): prevent crash when User-Agent header is missing

When anonymous access is enabled, session-less requests fall through to
the (app) layout's mobile-detection check instead of redirecting to
/login. Requests without a User-Agent header (e.g. proxy/health-check
probes) caused getSelectorsByUserAgent('') to return undefined, throwing
a TypeError on the isMobile destructure and crashing the web UI.

Only call getSelectorsByUserAgent when a non-empty User-Agent is present.

Fixes #1308

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: brendan-kellam

packages/web/src/app/(app)/layout.tsx

@@ -148,7 +148,7 @@ export default async function Layout(props: LayoutProps) {
     const headersList = await headers();
     const cookieStore = await cookies()
     const userAgent = headersList.get('user-agent');
-    const { isMobile } = getSelectorsByUserAgent(userAgent ?? '');
+    const { isMobile } = userAgent ? getSelectorsByUserAgent(userAgent) : { isMobile: false };
 
     if (isMobile && !cookieStore.has(MOBILE_UNSUPPORTED_SPLASH_SCREEN_DISMISSED_COOKIE_NAME)) {
         return (