feat(android): on-device OS notification capture (NotificationListenerService)

Add an Android backend to os_notification_listener.py so notifications on the
experimental Android-TV build drive the existing NotificationColorStripSource
LED effects (flash/pulse/sweep, per-app colors + sounds) at app-name parity
with the Windows/Linux backends.

A Kotlin NotificationListenerService forwards the posting app's display label
across the Chaquopy JNI boundary into a new push-based _AndroidBackend +
module-level push_notification() receiver; the existing color-strip pipeline,
per-app colors/filters, and history endpoint are reused unchanged.

- Python: _AndroidBackend (probed first), push_notification() receiver,
  _LinuxBackend.probe() hardened with is_linux() to exclude Android (which
  also reports platform.system() == "Linux").
- Android: LedGrabNotificationListener NLS — serial single-thread executor,
  full crash isolation around Python.getInstance(), label-only forwarding
  (never notification title/body), ongoing/group-summary/self-package noise
  filtering. Manifest service exported + gated by
  BIND_NOTIFICATION_LISTENER_SERVICE (no new uses-permission).
- UX: prompt-once notification-access + manual "Grant notification access"
  button wired into the D-pad focus chain (computed from visible controls);
  en/ru/zh strings.
- Tests: 11 isolated unit tests — module-global + tmp_path history isolation,
  push routing contract, callback-exception swallowing, None app-name, and a
  desktop-regression lock on backend selection order.
- Docs: README OS-support Android column (notification + audio cells),
  ANDROID-REVIEW status flipped to Implemented.

Zero new Python deps; no build.gradle.kts / Chaquopy pip changes.

android/app/src/main/AndroidManifest.xml

@@ -110,6 +110,23 @@
                 android:value="Root-mode screen capture for ambient LED sync. Uses /system/bin/screenrecord on rooted devices to avoid MediaProjection's persistent capture indicator overlay, which is required for the always-on ambient-lighting use case." />
         </service>
 
+        <!-- Notification capture — a NotificationListenerService bound by
+             system_server. exported="true" is REQUIRED here (the system binds
+             it cross-process) and intentionally diverges from CaptureService
+             (exported="false"); access is gated by the system-held
+             BIND_NOTIFICATION_LISTENER_SERVICE permission, so no new
+             <uses-permission> is needed. The user grants access via
+             Settings > Notification access (opened from MainActivity). -->
+        <service
+            android:name=".LedGrabNotificationListener"
+            android:label="@string/notification_listener_label"
+            android:exported="true"
+            android:permission="android.permission.BIND_NOTIFICATION_LISTENER_SERVICE">
+            <intent-filter>
+                <action android:name="android.service.notification.NotificationListenerService" />
+            </intent-filter>
+        </service>
+
         <!-- Autostart — fires on device boot (and package replace).
              On rooted devices, launches CaptureService directly so capture
              resumes without the user tapping Start. Unrooted devices are