refactor(dev): move DemoPanel into src/dev

DemoPanel is dev-only tooling like the rest of the UI Lab mock code, so it
lives with it now instead of among the real UI components. Import path in
App.tsx updated; the DEV gating (and production tree-shaking) is unchanged.

src/App.tsx

@@ -15,7 +15,7 @@ import { UpdateToast } from './components/UpdateToast'
 import { WhatsNewToast } from './components/WhatsNewToast'
 import { WhatsNewModal } from './components/WhatsNewModal'
 import { OnboardingOverlay } from './components/onboarding/OnboardingOverlay'
-import { DemoPanel } from './components/DemoPanel'
+import { DemoPanel } from './dev/DemoPanel'
 import { initializeNotifications } from './notifications'
 
 export default function App() {