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>

web/push_sw.js

@@ -0,0 +1,64 @@
+// Version: 1.0.0 | Created: 2026-04-27
+// Web Push service worker for M8Chat.
+// Receives push events from the browser and shows system notifications.
+// This script is registered separately from flutter_service_worker.js.
+
+'use strict';
+
+self.addEventListener('push', function (event) {
+  if (!event.data) return;
+
+  let data = {};
+  try {
+    data = event.data.json();
+  } catch (_) {
+    data = { notification: { body: event.data.text() } };
+  }
+
+  // Sygnal delivers: { notification: { content: { body, msgtype, ... }, room_name, sender_display_name, ... } }
+  const notif = data.notification || {};
+  const content = notif.content || {};
+
+  const sender = notif.sender_display_name || notif.sender || 'M8Chat';
+  const room = notif.room_name || notif.room_alias || '';
+  const body = content.body || notif.body || 'New message';
+
+  const title = room ? `${sender} in ${room}` : sender;
+
+  const options = {
+    body: body,
+    icon: '/icons/Icon-192.png',
+    badge: '/icons/Icon-192.png',
+    tag: notif.room_id || 'm8chat',
+    data: {
+      roomId: notif.room_id,
+      url: self.location.origin + (notif.room_id
+        ? '/rooms/' + encodeURIComponent(notif.room_id)
+        : '/rooms'),
+    },
+    requireInteraction: false,
+  };
+
+  event.waitUntil(self.registration.showNotification(title, options));
+});
+
+self.addEventListener('notificationclick', function (event) {
+  event.notification.close();
+
+  const url = (event.notification.data && event.notification.data.url)
+    ? event.notification.data.url
+    : self.location.origin + '/rooms';
+
+  event.waitUntil(
+    clients.matchAll({ type: 'window', includeUncontrolled: true }).then(function (clientList) {
+      for (const client of clientList) {
+        if (client.url === url && 'focus' in client) {
+          return client.focus();
+        }
+      }
+      if (clients.openWindow) {
+        return clients.openWindow(url);
+      }
+    })
+  );
+});