feat(eq): restyle equalizer screen and add custom presets

Restyle the Ecualizador settings screen to the new visual language while
keeping the equalizer at 5 bands (spike-resolved, Engram id 2498 - band
count is device-reported via just_audio's AndroidEqualizer, not app-chosen;
the approved mockup's 7 sliders would silently no-op on typical hardware).

- Restyle EcualizadorWidget in place: strip its internal title + preset
  chip row (the pushed screen's header now carries the title), add a
  habilitado parameter that greys/disables every slider when EQ is off.
  Widen PresetsEcualizadorWidget additively (personalizados param) so
  custom presets can join the chip row without a second implementation.
- Add servicio_presets_personalizados.dart (new file, own SharedPreferences
  key eq_custom_presets_v1) for custom EQ preset persistence - kept out of
  servicio_ecualizador.dart, which has an empty-git-diff success criterion
  for this change. preset_ecualizador.dart is unchanged: a custom preset is
  just a PresetEcualizador with a user-supplied name.
- Extend EstadoEcualizador with presetsPersonalizados,
  guardarPresetPersonalizado (validates non-empty name),
  eliminarPresetPersonalizado. The load is a new explicit
  cargarPresetsPersonalizados(), deliberately NOT folded into
  cargarPersistido(): that method is exercised ~30 times by
  estado_ecualizador_test.dart (protected, must stay unmodified) via Fakes
  only, with no SharedPreferences awareness in that file.
- Build out the Ecualizador screen body: base-vs-per-station explainer
  banner, a "Salida activa" row surfaced on the main screen (previously
  Advanced-only), an "Emisoras con ajuste propio" drill-down sourced from
  the existing presetsPorEmisora map, and a "Guardar como preset" action.

New coverage lives in new files rather than touching the three protected
EQ test files: ecualizador_widget_test.dart (component-level, did not
exist before this commit), servicio_presets_personalizados_test.dart, and
estado_ecualizador_presets_personalizados_test.dart. servicio_ecualizador.dart,
servicio_audio.dart and the three protected EQ test files keep an empty
git diff. Full suite: 713/713 green (2 skipped, unchanged), up from 682.

size:exception - realized 1,954 changed lines (25 files, plus this docs
update) against the 400-550 forecast: lib/ + ARB alone is ~650 lines, near
the top of the forecast band by itself since this WU also had to build out
a screen body WU3a only stubbed; the rest is 4 test files (675 lines) and
11 new ARB keys regenerating 13 lib/l10n/gen files (~546 lines) - the same
pattern every prior work unit in this branch has hit. Not splittable: WU14
reuses this unit's editor component by exact runtime type and cannot begin
until this lands as a whole.

test/servicios/servicio_presets_personalizados_test.dart

@@ -0,0 +1,126 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pluriwave/modelos/preset_ecualizador.dart';
+import 'package:pluriwave/servicios/servicio_presets_personalizados.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// WU13 task 13.2 — persistence for user-named custom EQ presets.
+///
+/// Design ADR-5 hazard box: this lives in its OWN file and OWN
+/// SharedPreferences key (`eq_custom_presets_v1`), deliberately separate
+/// from `ServicioEcualizador` — that file has an empty-`git diff` success
+/// criterion for this change, so persistence for custom presets must never
+/// be added there.
+void main() {
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+  });
+
+  test('listar returns an empty list when nothing was saved yet', () async {
+    final prefs = await SharedPreferences.getInstance();
+    final servicio = ServicioPresetsPersonalizados(prefs: prefs);
+
+    expect(await servicio.listar(), isEmpty);
+  });
+
+  test('guardar then listar round-trips a named custom preset', () async {
+    final prefs = await SharedPreferences.getInstance();
+    final servicio = ServicioPresetsPersonalizados(prefs: prefs);
+    // Not `const`: PresetEcualizador's constructor asserts `bandas.length
+    // == 5`, and `List.length` is not constant-foldable in an assert here
+    // (same const-eval limitation the branch already hit for
+    // `DateTime(...)` fixtures in WU9) — the model's own static presets
+    // (e.g. `PresetEcualizador.flat`) are declared `final`, not `const`,
+    // for the same reason.
+    final preset = PresetEcualizador(
+      nombre: 'Mi preset',
+      bandas: [1.0, -2.0, 3.0, -4.0, 5.0],
+    );
+
+    await servicio.guardar(preset);
+    final guardados = await servicio.listar();
+
+    expect(guardados, hasLength(1));
+    expect(guardados.single, equals(preset));
+  });
+
+  test('guardar persists under its OWN SharedPreferences key', () async {
+    final prefs = await SharedPreferences.getInstance();
+    final servicio = ServicioPresetsPersonalizados(prefs: prefs);
+    final preset = PresetEcualizador(nombre: 'Otro', bandas: [0, 0, 0, 0, 0]);
+
+    await servicio.guardar(preset);
+
+    expect(prefs.getString('eq_custom_presets_v1'), isNotNull);
+  });
+
+  test('guardar with a repeated name replaces the previous entry', () async {
+    final prefs = await SharedPreferences.getInstance();
+    final servicio = ServicioPresetsPersonalizados(prefs: prefs);
+    final v1 = PresetEcualizador(nombre: 'Mismo', bandas: [1, 1, 1, 1, 1]);
+    final v2 = PresetEcualizador(nombre: 'Mismo', bandas: [2, 2, 2, 2, 2]);
+
+    await servicio.guardar(v1);
+    await servicio.guardar(v2);
+    final guardados = await servicio.listar();
+
+    expect(guardados, hasLength(1));
+    expect(guardados.single.bandas, equals(v2.bandas));
+  });
+
+  test('guardar preserves insertion order across multiple presets', () async {
+    final prefs = await SharedPreferences.getInstance();
+    final servicio = ServicioPresetsPersonalizados(prefs: prefs);
+    final primero = PresetEcualizador(
+      nombre: 'Primero',
+      bandas: [0, 0, 0, 0, 0],
+    );
+    final segundo = PresetEcualizador(
+      nombre: 'Segundo',
+      bandas: [0, 0, 0, 0, 0],
+    );
+
+    await servicio.guardar(primero);
+    await servicio.guardar(segundo);
+    final guardados = await servicio.listar();
+
+    expect(guardados.map((p) => p.nombre).toList(), ['Primero', 'Segundo']);
+  });
+
+  test('eliminar removes the named preset only', () async {
+    final prefs = await SharedPreferences.getInstance();
+    final servicio = ServicioPresetsPersonalizados(prefs: prefs);
+    final conservar = PresetEcualizador(
+      nombre: 'Conservar',
+      bandas: [0, 0, 0, 0, 0],
+    );
+    final borrar = PresetEcualizador(nombre: 'Borrar', bandas: [0, 0, 0, 0, 0]);
+    await servicio.guardar(conservar);
+    await servicio.guardar(borrar);
+
+    await servicio.eliminar('Borrar');
+    final guardados = await servicio.listar();
+
+    expect(guardados, hasLength(1));
+    expect(guardados.single.nombre, equals('Conservar'));
+  });
+
+  test('eliminar on an unknown name is a safe no-op', () async {
+    final prefs = await SharedPreferences.getInstance();
+    final servicio = ServicioPresetsPersonalizados(prefs: prefs);
+
+    await servicio.eliminar('no existe');
+
+    expect(await servicio.listar(), isEmpty);
+  });
+
+  test(
+    'a corrupt top-level payload degrades to an empty list, never throws',
+    () async {
+      final prefs = await SharedPreferences.getInstance();
+      await prefs.setString('eq_custom_presets_v1', 'not valid json{{{');
+      final servicio = ServicioPresetsPersonalizados(prefs: prefs);
+
+      await expectLater(servicio.listar(), completion(isEmpty));
+    },
+  );
+}