feat(alarm): make the ring immune to device media volume

The alarm's steady-state audio runs on the Flutter media-stream
player after the native handoff, so device volume 0 silenced it
entirely. The ring now forces STREAM_MUSIC to an audible reference:
Dart requests the override before pre-starting alarm audio (fallback
WAV included), Kotlin captures the current volume once and restores
it idempotently on every exit path (dismiss, snooze, dispose), with
a native best-effort backstop in service teardown.

The backstop is handoff-aware via PluriWaveAlarmService.flutterOwnsRing:
confirmFlutterAudio marks the handoff before triggering the native
stop, so the backstop cannot restore the volume mid-ring right as the
Flutter player takes over (that would re-silence the alarm at volume
0). The flag resets at every ring start; Flutter process death after
handoff remains a documented best-effort gap.

The alarm's perceived loudness keeps ramping 5% to the configured
volume through the player as before; normal radio playback and call
ducking never touch the override.

Work unit 2/3 of alarm-volume-ramp-restore (ring volume override).

lib/app.dart

@@ -363,6 +363,14 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal> {
   }
 
   Future<void> _prearrancarAudioAlarma(AlarmaMusical alarma) async {
+    // Must run FIRST, before any early return: the override needs to be in
+    // effect for the whole ring, including fallback-WAV-only alarms that
+    // never reach the station-playback branch below (Requirement:
+    // Ring-scoped device-volume override).
+    await context.read<EstadoAlarmas>().android.forzarVolumenMediaParaAlarma(
+      1.0,
+    );
+    if (!mounted) return;
     final emisora = alarma.emisora;
     if (emisora == null) return;
     final radio = context.read<EstadoRadio>();