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>

README.md

@@ -4,7 +4,7 @@ A self-hosted second brain and project management application with integrated LL
 
 ## Features
 
-Notes and tasks with a Markdown editor, sub-tasks, milestones, and kanban project workspaces. AI chat with streaming responses, RAG over your notes, and tool use (web search, calendar, weather). A daily briefing that digests your tasks, RSS feeds, and weather on a schedule. Knowledge graph, per-user/group sharing, PWA with push notifications, an MCP server for external AI clients, and an Android companion app.
+Notes and tasks with a Markdown editor, sub-tasks, milestones, and kanban project workspaces. AI chat with streaming responses, RAG over your notes, and tool use (web search, calendar, weather). A daily briefing that digests your tasks, RSS feeds, and weather on a schedule. Knowledge graph, per-user/group sharing, PWA with push notifications, and an MCP server for external AI clients.
 
 ## Quick Start
 
@@ -36,7 +36,6 @@ Open `http://localhost:5000`. The first user to register becomes admin. Go to **
 | [API Keys & MCP](docs/api-keys-and-mcp.md) | API key management and Fable MCP install guide |
 | [SSO / OAuth](docs/sso-oauth.md) | OIDC setup for Authentik, Keycloak, and other providers |
 | [API Reference](docs/api-reference.md) | All REST API endpoints |
-| [Android App](docs/android-app.md) | Flutter companion app architecture and feature status |
 
 ## License