feat: Add Chat UX improvements with notifications and @mention support

- Add ActionBar component with expandable toolbar for mobile
- Add @mention functionality with autocomplete dropdown
- Add browser notification system (push, sound, vibration)
- Add NotificationSettings modal for user preferences
- Add mention badges on room list cards
- Add ReportPreview with Markdown rendering and copy/download
- Add message copy functionality with hover actions
- Add backend mentions field to messages with Alembic migration
- Add lots field to rooms, remove templates
- Optimize WebSocket database session handling
- Various UX polish (animations, accessibility)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/package.json

@@ -17,7 +17,9 @@
     "axios": "^1.13.2",
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
+    "react-markdown": "^10.1.0",
     "react-router": "^7.9.6",
+    "remark-gfm": "^4.0.1",
     "zustand": "^5.0.9"
   },
   "devDependencies": {