Add docs/mobile-app-plan.md capturing the mobile-app research: current
stack, existing responsive web inventory, why the editor must stay in a
WebView, comparison of native/Capacitor/hybrid paths, the recommended
Capacitor route, required backend changes (token-in-body login, CORS
whitelist, APNs/FCM push, optional Swagger), Android/iOS specifics, and
the offline outlook (referencing docs/offline-sync-plan.md).
Enrich the "Mobile app" roadmap entry in README.md and README.ru.md to
point to the new plan and correct the inaccurate "native" wording
(Capacitor wrapper of the existing web UI, iOS first, Android to follow).
1207ef1143