feat: web push notifications, image fullscreen, public room browser

- Web Push: push_sw.js service worker + VAPID subscription registered
  on login; Matrix pusher registered pointing to Sygnal on
  https://matrix.m8chat.au/_matrix/push/v1/notify
- Chat: tap image -> fullscreen InteractiveViewer (pinch/zoom)
- Rooms: public room browser via Matrix room directory (FAB + empty state)

Build fixes applied to web_push_service.dart:
  - dart:js_interop_unsafe import added for getProperty on JSObject
  - client.setPusher -> client.postPusher (matrix 0.33.0 API)
  - keyBytes.toJS removed (JSUint8Array already a JS type)
  - register() script URL coerced with .toJS (String -> JSAny)

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: 1.4.0+6
+version: 1.5.0+7
 
 environment:
   sdk: '>=3.11.0 <4.0.0'