fix(ecualizador): header switch, banner radius, preset chips, glow thumb

Audit 11.1 (t4:566): the master enable switch moves to
PluriPushScaffold's header actions -- was the body's first
SwitchListTile row. Its realtime/pending explainer subtitle stays
behind as a plain caption so no information is lost.

Audit 11.2 (t4:571): explainer banner radius is 16 (a local one-off,
matching neither of the 3 named tokens), not radiusSm's 14.

Audit 11.3 (t4:574-577): preset chips are solid brand-teal with dark
text when active, listSurface + a faint border when not -- was
Material's own ChoiceChip theming (primaryContainer/grey).

Audit 11.5 (t4:585): band sliders use a new 20x20 _GlowSliderThumbShape
(brand-teal blurred glow + solid thumb), replacing the Material
default round thumb.

Audit 11.7 (t4:584): the dB label is brand teal at 90% alpha, not
liveGreen -- a leftover wrong colour family 11.6's slider-only fix
never touched.

Updated 2 pre-existing tests (pantalla_ajustes_ecualizador_test.dart,
pantalla_ajustes_test.dart) to locate the enable switch by key instead
of by the "Enable equalizer" text it no longer renders next to.

test/pantallas/pantalla_ajustes_test.dart

@@ -162,9 +162,10 @@ void main() {
       await pumpStable(tester);
 
       // Pushed, not index-switched: exactly one PluriPushScaffold now exists,
-      // and its moved control (the enable switch) is reachable.
+      // and its moved control (the enable switch, audit 11.1 -- now a
+      // header action, t4 line 566) is reachable.
       expect(find.byType(PluriPushScaffold), findsOneWidget);
-      expect(find.text('Enable equalizer'), findsOneWidget);
+      expect(find.byKey(const ValueKey('eq-master-switch')), findsOneWidget);
     });
 
     testWidgets('tapping the Orden de listas row pushes its detail screen', (