fix: sync mobile routing with browser URL for direct navigation (refs #163)

URL-to-screen sync on mount and screen-to-URL sync via replaceState
enable direct URL navigation, page refresh, and bookmarks on mobile.

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

frontend/src/core/store/index.ts

@@ -1,5 +1,5 @@
 // Export navigation store
-export { useNavigationStore } from './navigation';
+export { useNavigationStore, routeToScreen, screenToRoute } from './navigation';
 export type { MobileScreen, VehicleSubScreen } from './navigation';
 
 // Export user store