docs(mobile): add mobile app plan and link it from README roadmap

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).

README.md

@@ -110,7 +110,7 @@ community feature, with no enterprise license. Open it from the page header; the
 - 🔭 **Viewer comments** — let read-only viewers leave comments.
 - 🔭 **Password-protected pages** — protect individual pages / shares with a password.
 - 🔭 **Windows / Linux app** — native desktop app for Windows and Linux.
-- 🔭 **Mobile app** — native mobile application.
+- 🔭 **Mobile app** — mobile apps (iOS first, Android to follow), reusing the existing responsive web UI and editor via a Capacitor wrapper, with offline planned for later. See [docs/mobile-app-plan.md](docs/mobile-app-plan.md).
 - 🔭 **Offline mode** — offline sync & PWA support.
 - 🔭 **Voice dictation** — microphone button in the AI agent chat and the page editor; audio is transcribed server-side (Whisper / OpenAI-compatible STT) via the workspace AI provider, with an admin toggle to show/hide it. See [docs/voice-dictation-plan.md](docs/voice-dictation-plan.md).
 - 🔭 **Editor & UX improvements** — blocks inside tables (lists, to-do items), column layout, additional heading levels, highlight blocks, custom emoji in callouts, floating images, anchor links for page mentions, toggles (shared-page width, aside/sidebar, spellcheck, ligatures), sanitized space-tree export, and mentions in breadcrumbs.