Reach the screen in the background, and leave it again

Measured on an Android 15 emulator with the app backgrounded and the display
asleep: the siren played, the platform refused the takeover
("Background activity launch blocked! … BAL_BLOCK"), and the alarm was
destroyed mid-ring by its own receipt ack.

Takeover:
- the foreground service posts an alarm_firing notification whose
  full-screen intent raises AlarmActivity — the only background launch
  Android 14+ permits. Verified with the display asleep: wakefulness
  Asleep → Awake, topResumedActivity=.AlarmActivity, the FSI on the record.
- the cancel lives on the four real silences (disarm, stop, JS ack, activity
  destroy), not in AlarmPlayer.stop: play() calls stop() first, which
  retracted the notification ~120 ms after posting it.
- "Back to alerts" now finishes the alarm window (AlarmModule
  finishTakeover). A route pop is not an activity: the window stayed on top
  for good with show-when-locked, keep-screen-on and no Recents entry — the
  reported "no way back to the main menu".

The native-delivery contract test said "no notification ever announces an
alarm"; that was falsified on device, so it now pins one FSI notification and
a silent firing channel instead.

Verified end to end on the emulator (two runs, one with the screen off):
join without an account, armed service, background trigger → wake + takeover
+ siren, acknowledge → silence, notification retracted, alert delivered,
takeover closed by itself, Home reachable, reopening silent.

android/app/src/main/java/com/dcntrlized/alarm/AlarmActivity.kt

@@ -7,6 +7,7 @@ import android.content.Intent
 import android.os.Build
 import android.os.Bundle
 import android.view.WindowManager
+import java.lang.ref.WeakReference
 import com.facebook.react.ReactActivity
 import com.facebook.react.ReactActivityDelegate
 import com.facebook.react.defaults.DefaultNewArchitectureEntryPoint.fabricEnabled
@@ -38,6 +39,7 @@ class AlarmActivity : ReactActivity() {
         // Order matters: the flags must be set before the window is shown.
         showOverLockScreen()
         super.onCreate(savedInstanceState)
+        current = WeakReference(this)
     }
 
     private fun showOverLockScreen() {
@@ -67,11 +69,30 @@ class AlarmActivity : ReactActivity() {
     }
 
     override fun onDestroy() {
+        if (current?.get() === this) current = null
         AlarmPlayer.stop(this)
+        // The ringing notification dies with the ring it announced. NOT in
+        // AlarmPlayer.stop: play() calls that first, which would retract the
+        // full-screen intent that raised this activity.
+        AlarmNotifier.cancelFiring(this)
         super.onDestroy()
     }
 
     companion object {
+        /**
+         * The takeover that is on screen, if any.
+         *
+         * The JS "Back to alerts" action pops the route but cannot dismiss an
+         * Android activity, so without this the alarm window would stay on top
+         * for good — visible over the lock screen, holding the screen awake, in a
+         * task that is excluded from Recents. That is the reported "no way back to
+         * the main menu". [AlarmModule.finishTakeover] ends it.
+         */
+        @Volatile
+        private var current: WeakReference<AlarmActivity>? = null
+
+        fun currentActivity(): AlarmActivity? = current?.get()
+
         const val EXTRA_ALERT_ID = "alarm.alertId"
         const val EXTRA_MESSAGE = "alarm.message"