notifications + window: add Android native plugins

`ux/notifications/events` and `ux/window/events` only had macOS stream
handlers, so on Android/iOS the unconditional Dart subscription threw
MissingPluginException at startup (EventChannel reports activation
failures straight to FlutterError.onError, bypassing the `onError:`
callback).

- Gate each Dart event-channel subscription to platforms that register a
  native handler (`defaultTargetPlatform`), silencing iOS.
- `WindowPlugin`: report app foreground/background as host focus via
  `ProcessLifecycleOwner` ON_START/ON_STOP, so a backgrounded-but-alive
  process reports `focused = false`.
- `NotificationsPlugin`: local notifications (show/cancel by thread/all),
  POST_NOTIFICATIONS request on 13+, and tap routing back over the event
  channel — a tap that cold-starts the process is buffered until Dart
  subscribes.
- Regression tests for the subscription gate plus contract tests for the
  method/event payloads.

android/src/main/AndroidManifest.xml

@@ -1,6 +1,9 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="io.swipelab.ux">
 
+  <!-- NotificationsPlugin posts notifications on Android 13+. -->
+  <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
+
   <application>
     <provider
       android:name="androidx.core.content.FileProvider"