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.

server/src/ledgrab/core/processing/os_notification_listener.py

@@ -8,6 +8,8 @@ Supported platforms:
 - **Windows**: polls toast notifications via winrt UserNotificationListener
   (falls back to winsdk if winrt packages are not installed)
 - **Linux**: monitors org.freedesktop.Notifications via D-Bus (dbus-next)
+- **Android**: receives notifications pushed from a Kotlin NotificationListenerService
+  via Chaquopy (push-based; see push_notification() and _AndroidBackend)
 """
 
 import asyncio
@@ -17,9 +19,10 @@ import platform
 import threading
 import time
 from pathlib import Path
-from typing import Dict, List, Optional, Set
+from typing import Callable, Dict, List, Optional, Set
 
 from ledgrab.utils import get_logger
+from ledgrab.utils.platform import is_linux
 
 logger = get_logger(__name__)
 
@@ -30,15 +33,71 @@ _HISTORY_MAX = 50
 # Module-level singleton for dependency access
 _instance: Optional["OsNotificationListener"] = None
 
+# Push target for the Android backend — set by _AndroidBackend.start(), read by
+# push_notification(). None when the Android backend isn't running (desktop / server down).
+_android_target: Callable[[str | None], None] | None = None
+
 
 def get_os_notification_listener() -> Optional["OsNotificationListener"]:
     """Return the global OsNotificationListener instance (or None)."""
     return _instance
 
 
+def push_notification(app_name: str | None) -> None:
+    """Receive an Android notification pushed from Kotlin via Chaquopy.
+
+    Called by the LedGrabNotificationListener service through
+    ``Python.getInstance().getModule(...).callAttr("push_notification", label)``.
+    Routes the posting app's display label into the active listener's
+    ``_on_new_notification`` handler. No-op when the Android backend isn't running,
+    so a notification arriving before the server is ready (or on desktop) is safely
+    ignored.
+    """
+    # Snapshot into a local first: stop() may null _android_target concurrently, but an
+    # in-flight push then still completes against the prior callback. Do NOT collapse this
+    # into `if _android_target is not None: _android_target(...)` — that reintroduces a
+    # TOCTOU None-deref race.
+    cb = _android_target
+    if cb is None:
+        return
+    try:
+        cb(app_name)
+    except Exception as exc:  # never let a JNI-side call crash the bound service
+        logger.warning("push_notification callback error: %s", exc)
+
+
 # ── Platform backends ──────────────────────────────────────────────────
 
 
+class _AndroidBackend:
+    """Push-based backend — notifications arrive from Kotlin via push_notification().
+
+    Unlike the Windows/Linux backends (which poll or eavesdrop on a thread), Android
+    notifications are delivered by a Kotlin NotificationListenerService across the
+    Chaquopy JNI boundary into the module-level push_notification() receiver, so
+    start()/stop() simply register/clear the receiver target.
+    """
+
+    def __init__(self, on_notification):
+        self._on_notification = on_notification
+
+    @staticmethod
+    def probe() -> bool:
+        """Return True when running on Android (Chaquopy)."""
+        from ledgrab.utils.platform import is_android
+
+        return is_android()
+
+    def start(self) -> None:
+        global _android_target
+        _android_target = self._on_notification
+        logger.info("OS notification listener: Android backend active")
+
+    def stop(self) -> None:
+        global _android_target
+        _android_target = None
+
+
 def _import_winrt_notifications():
     """Try to import WinRT notification APIs: winrt first, then winsdk fallback.
 
@@ -193,7 +252,9 @@ class _LinuxBackend:
     @staticmethod
     def probe() -> bool:
         """Return True if this backend can run on the current system."""
-        if platform.system() != "Linux":
+        # is_linux() excludes Android, which also reports platform.system() == "Linux"
+        # but has no D-Bus session — defense-in-depth beyond probe ordering.
+        if not is_linux():
             return False
         try:
             import dbus_next  # noqa: F401
@@ -312,8 +373,9 @@ class OsNotificationListener:
         global _instance
         _instance = self
 
-        # Try platform backends in order
-        for backend_cls in (_WindowsBackend, _LinuxBackend):
+        # Try platform backends in order (Android first — it reports platform.system()
+        # == "Linux", so probing it ahead of _LinuxBackend is the robust ordering).
+        for backend_cls in (_AndroidBackend, _WindowsBackend, _LinuxBackend):
             if backend_cls.probe():
                 self._backend = backend_cls(on_notification=self._on_new_notification)
                 self._backend.start()