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/res/layout/activity_main.xml

@@ -66,6 +66,21 @@
             android:focusableInTouchMode="true"
             android:nextFocusDown="@+id/autostart_check" />
 
+        <!-- Shown only while notification-listener access is missing. The D-pad
+             focus chain is wired at runtime (wireStoppedFocusChain) because this
+             button and the autostart checkbox are both conditionally visible. -->
+        <Button
+            android:id="@+id/grant_notification_button"
+            style="@style/Widget.LedGrab.Button.Secondary"
+            android:layout_width="320dp"
+            android:layout_height="56dp"
+            android:layout_marginTop="20dp"
+            android:text="@string/btn_grant_notification_access"
+            android:textSize="18sp"
+            android:focusable="true"
+            android:focusableInTouchMode="true"
+            android:visibility="gone" />
+
         <CheckBox
             android:id="@+id/autostart_check"
             android:layout_width="wrap_content"

android/app/src/main/res/values-ru/strings.xml

@@ -25,4 +25,6 @@
     <string name="notification_channel_description">Отображается, пока LedGrab захватывает экран.</string>
     <string name="notification_title">LedGrab работает</string>
     <string name="notification_text">Веб-интерфейс: %1$s</string>
+    <string name="notification_listener_label">Захват уведомлений LedGrab</string>
+    <string name="btn_grant_notification_access">Разрешить доступ к уведомлениям</string>
 </resources>

android/app/src/main/res/values-zh/strings.xml

@@ -25,4 +25,6 @@
     <string name="notification_channel_description">LedGrab 捕获屏幕时显示。</string>
     <string name="notification_title">LedGrab 运行中</string>
     <string name="notification_text">Web界面:%1$s</string>
+    <string name="notification_listener_label">LedGrab 通知捕获</string>
+    <string name="btn_grant_notification_access">授予通知访问权限</string>
 </resources>

android/app/src/main/res/values/strings.xml

@@ -25,4 +25,6 @@
     <string name="notification_channel_description">Shows while LedGrab is capturing the screen.</string>
     <string name="notification_title">LedGrab Running</string>
     <string name="notification_text">Web UI: %1$s</string>
+    <string name="notification_listener_label">LedGrab notification capture</string>
+    <string name="btn_grant_notification_access">Grant notification access</string>
 </resources>