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.

test/pantallas/pantalla_alarma_sonando_scaffold_test.dart

@@ -91,7 +91,6 @@ Future<void> _montarPantalla(
         builder:
             (_) => PantallaAlarmaSonando(
               alarma: estadoAlarmas.alarmas.single,
-              audioPrearrancado: true,
             ),
         fullscreenDialog: true,
       ),