Apply theme/custom CSS in popped-out standalone tabs (ticket 025)

StandalonePage now calls useSettingsPanel() so colour scheme and any
active custom theme CSS are applied on mount and stay in sync with
settings changes, matching the main window's NavMenu-driven behavior.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/App.tsx

@@ -19,6 +19,7 @@ import {
   type TimelineBucket,
 } from './lib/api'
 import { DocsNavProvider } from './lib/docsNav'
+import { useSettingsPanel } from './lib/useSettingsPanel'
 
 const POLL_MS = 2000
 
@@ -34,7 +35,12 @@ function parsePopOutQuery(): { section: PopOutSectionId | null; range: TimeRange
   return { section, range }
 }
 
+// Popped-out routes render outside NavMenu, which is otherwise the only place that calls `useSettingsPanel()` and applies the
+// colour scheme / custom theme CSS as a side effect. Calling it here (state unused) keeps standalone tabs themed the same as
+// the main window, including staying live if settings change while the tab is open.
 function StandalonePage({ children }: { children: ReactNode }) {
+  useSettingsPanel()
+
   return (
     <DocsNavProvider>
       <div className="h-screen w-screen overflow-auto">{children}</div>