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.

lib/servicios/servicio_presets_personalizados.dart

@@ -0,0 +1,88 @@
+import 'dart:convert';
+
+import 'package:shared_preferences/shared_preferences.dart';
+
+import '../modelos/preset_ecualizador.dart';
+import 'persistencia_tolerante.dart';
+
+/// Persistence for USER-NAMED custom EQ presets (design ADR-5 hazard box,
+/// `eq-custom-presets` spec — "Custom Preset Save").
+///
+/// Deliberately its OWN file and OWN SharedPreferences key
+/// (`eq_custom_presets_v1`), never folded into [ServicioEcualizador]: that
+/// file has an empty-`git diff` success criterion for this change, so
+/// nothing custom-preset-related may be added there. A custom preset is
+/// just a [PresetEcualizador] with a user-supplied `nombre` — the model
+/// itself needs no change (`toJson`/`desdeJson` already exist).
+class ServicioPresetsPersonalizados {
+  ServicioPresetsPersonalizados({SharedPreferences? prefs}) : _prefs = prefs;
+
+  static const _keyPresetsPersonalizados = 'eq_custom_presets_v1';
+
+  final SharedPreferences? _prefs;
+
+  Future<SharedPreferences> _resolverPrefs() async =>
+      _prefs ?? SharedPreferences.getInstance();
+
+  /// Returns every saved custom preset, in save order.
+  Future<List<PresetEcualizador>> listar() async {
+    final prefs = await _resolverPrefs();
+    return _leer(prefs);
+  }
+
+  /// Saves [preset] under its own name. A preset already saved under the
+  /// same name is replaced (last write wins) rather than duplicated.
+  Future<void> guardar(PresetEcualizador preset) async {
+    final prefs = await _resolverPrefs();
+    final actuales =
+        _leer(prefs)
+          ..removeWhere((p) => p.nombre == preset.nombre)
+          ..add(preset);
+    await _guardarTodos(prefs, actuales);
+  }
+
+  /// Removes the custom preset named [nombre], if present. A safe no-op
+  /// when no preset with that name exists.
+  Future<void> eliminar(String nombre) async {
+    final prefs = await _resolverPrefs();
+    final actuales = _leer(prefs)..removeWhere((p) => p.nombre == nombre);
+    await _guardarTodos(prefs, actuales);
+  }
+
+  /// Reads the persisted list with per-entry tolerance
+  /// (persistence-resilience D1/D6, same shared helper `ServicioEcualizador`
+  /// uses): an entry that fails to parse is skipped and logged, its
+  /// siblings survive. A top-level decode failure degrades to an empty
+  /// list (also logged) — custom presets are explicit-only user writes and
+  /// trivially re-creatable, so there is no flag/quarantine here, matching
+  /// `ServicioEcualizador`'s own documented D6 asymmetry vs. Alarms/Stations.
+  List<PresetEcualizador> _leer(SharedPreferences prefs) {
+    final raw = prefs.getString(_keyPresetsPersonalizados);
+    if (raw == null || raw.isEmpty) return [];
+    try {
+      final data = jsonDecode(raw) as List<dynamic>;
+      final resultado = parseListaTolerante<PresetEcualizador>(
+        data,
+        (entrada) => PresetEcualizador.desdeJson(entrada),
+        subsistema: 'presets_personalizados',
+        coleccion: _keyPresetsPersonalizados,
+      );
+      return resultado.validas;
+    } catch (e) {
+      registrarSaltoPersistencia(
+        subsistema: 'presets_personalizados',
+        detalle: _keyPresetsPersonalizados,
+        razon: e.toString(),
+      );
+      return [];
+    }
+  }
+
+  Future<void> _guardarTodos(
+    SharedPreferences prefs,
+    List<PresetEcualizador> presets,
+  ) async {
+    final serializado = presets.map((p) => p.toJson()).toList();
+    await prefs.setString(_keyPresetsPersonalizados, jsonEncode(serializado));
+  }
+}