Fix expired session showing EmbeddedFallback instead of login page

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

frontend/src/api.ts

@@ -11,6 +11,10 @@ async function request<T>(path: string, opts: RequestInit = {}): Promise<T> {
     headers: { 'Content-Type': 'application/json', ...opts.headers },
     ...opts,
   })
+  if (res.status === 401) {
+    ;(window.top ?? window).location.href = '/login'
+    throw new Error('Unauthenticated')
+  }
   if (!res.ok) {
     const err = await res.json().catch(() => ({ error: res.statusText }))
     throw new Error(err.error || `Request failed: ${res.status}`)