refactor(alarm): make the ringing screen pure UI over a reduced native port

PantallaAlarmaSonando no longer owns any audio orchestration (fallback
player, dB ramp, native handoff confirm, media-volume override/restore):
it only calls EstadoAlarmas.finalizarEjecucion/posponerAlarma from
Stop/Snooze/back, keeping the single-exit guard, PopScope back=Stop and
dismiss semantics intact. The status line now reads directly from the
alarm's static config (station name or a neutral label) instead of a
live playback/handoff state.

PuertoAlarmasAndroid drops confirmarAudioFlutter,
forzarVolumenMediaParaAlarma and restaurarVolumenMedia, and app.dart no
longer pre-starts a station before pushing the ring screen. This is the
Dart half of moving to a single native ring-audio owner (WU1 of 2); the
Kotlin service rebuild lands next and keeps this intermediate state
shippable with no double audio.

lib/app.dart

@@ -89,7 +89,6 @@ class _PaginaPrincipal extends StatefulWidget {
 }
 
 class _PaginaPrincipalState extends State<_PaginaPrincipal> {
-  static const _volumenInicialFadeInAlarmas = 0.05;
   int _indice = 0;
   StreamSubscription<String>? _errorSubscription;
   StreamSubscription<EventoAlarmaAndroid>? _alarmaSubscription;
@@ -353,15 +352,10 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal> {
     _alarmaSonandoId = alarma.id;
 
     try {
-      await _prearrancarAudioAlarma(alarma);
       if (!mounted) return;
       await Navigator.of(context).push(
         MaterialPageRoute<void>(
-          builder:
-              (_) => PantallaAlarmaSonando(
-                alarma: alarma,
-                audioPrearrancado: alarma.emisora != null,
-              ),
+          builder: (_) => PantallaAlarmaSonando(alarma: alarma),
           fullscreenDialog: true,
         ),
       );
@@ -373,23 +367,6 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal> {
     }
   }
 
-  Future<void> _prearrancarAudioAlarma(AlarmaMusical alarma) async {
-    // The ring-scoped stream override is NOT raised here anymore: the ringing
-    // screen owns it and raises it only when audio is actually about to be
-    // audible (first `reproduciendo`, or right before the fallback WAV
-    // plays). Raising it during pre-start let the ExoPlayer track spin-up —
-    // which briefly runs at gain 1.0 — blast at the ring level; under the
-    // user's original stream volume that spin-up is inaudible.
-    final emisora = alarma.emisora;
-    if (emisora == null) return;
-    final radio = context.read<EstadoRadio>();
-    debugPrint(
-      '[PluriWave][alarmas] prearrancar emisora alarma id=${alarma.id} emisora=${emisora.nombre}',
-    );
-    await radio.audio.setVolumen(_volumenInicialFadeInAlarmas);
-    unawaited(radio.reproducir(emisora));
-  }
-
   void _mostrarTimerDialog(BuildContext context) {
     showModalBottomSheet(
       context: context,