fix(alarma-sonando): restyle Stop as a neutral translucent surface

Audit 9.11 (t4 line 434): the Stop button was a FilledButton in
colorScheme.primary (brand cyan), the wrong colour family for the
largest element on the ringing screen. The prototype draws a neutral
rgba(255,255,255,.08) surface with a rgba(255,255,255,.16) border and
radius 24 (none of PluriWaveTokens' three named radii).

lib/pantallas/pantalla_alarma_sonando.dart

@@ -283,9 +283,22 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
                     child: FilledButton.icon(
                       key: const ValueKey('ringing-stop-button'),
                       style: FilledButton.styleFrom(
+                        // Audit 9.11 (t4 line 434): the prototype's Stop is a
+                        // NEUTRAL translucent surface — not the brand cyan
+                        // `colorScheme.primary` this used to render in. It is
+                        // the largest element on a full-screen surface, so
+                        // the wrong colour family was maximally visible.
+                        backgroundColor: Colors.white.withValues(alpha: 0.08),
+                        foregroundColor:
+                            Theme.of(context).colorScheme.onSurface,
+                        side: BorderSide(
+                          color: Colors.white.withValues(alpha: 0.16),
+                        ),
                         minimumSize: const Size.fromHeight(76),
                         shape: RoundedRectangleBorder(
-                          borderRadius: BorderRadius.circular(tokens.radiusLg),
+                          borderRadius: BorderRadius.circular(
+                            _stopButtonRadius,
+                          ),
                         ),
                       ),
                       onPressed: _detener,
@@ -341,6 +354,12 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
 String _hora(AlarmaMusical alarma) =>
     '${alarma.hora.toString().padLeft(2, '0')}:${alarma.minuto.toString().padLeft(2, '0')}';
 
+/// Audit 9.11 (t4 line 434): the Stop button's radius — doesn't match any of
+/// [PluriWaveTokens]'s three named radii (14/18/30), so it stays a local
+/// constant here rather than growing the shared token surface for a single
+/// call site (mirrors `_ArteEscuchar._radio` in `pantalla_inicio.dart`).
+const _stopButtonRadius = 24.0;
+
 /// Full-bleed blurred backdrop (WU11, replaces the `PluriGlassSurface` card
 /// container per task 11.3). This app has no per-station artwork/favicon
 /// safe to render here: `Emisora.favicon` is a network URL, and rendering