docs: remove Flutter companion app from project surface

The Flutter app (separate fabled_app repo) no longer adds value over
web/PWA access. Strip the in-repo surface that referenced it:

- delete docs/android-app.md
- drop README docs-table row and feature-list mention
- drop the two Flutter roadmap bullets from docs/features.md
- remove the Flutter port subsection from docs/design-system.md

The standalone fabled_app repo is untouched here; archival/deletion
of that repo is a separate decision.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/features.md

@@ -132,8 +132,6 @@ Settings are tabbed:
 
 - Email integration (read/send via IMAP/SMTP tools in chat)
 - Session invalidation on user deletion
-- Flutter push notifications (requires FCM/APNs — separate from web VAPID)
-- Flutter milestone support in project view
 
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