feat: image fullscreen viewer, public room browser

- Chat: tap any image to open fullscreen InteractiveViewer (pinch/zoom)
- Rooms: public room browser (Find rooms FAB + empty state button)
  searches Matrix room directory via queryPublicRooms, join from results
- Rooms: empty state now shows 'Find rooms' button to onboard new users

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pubspec.yaml

@@ -1,7 +1,7 @@
 name: m8chat_app
 description: "M8Chat — Matrix chat client for Android, iOS, and Web."
 publish_to: 'none'
-version: 1.3.1+5
+version: 1.4.0+6
 
 environment:
   sdk: '>=3.11.0 <4.0.0'