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/l10n/app_en.arb

@@ -95,6 +95,17 @@
   "equalizerPerStationTitle": "Use custom EQ for this favorite",
   "equalizerPerStationActive": "Active for {stationName}",
   "equalizerPerStationMain": "Using main EQ for {stationName}",
+  "equalizerBaseExplainer": "This is the base equalizer: it applies to every station without its own setting. A station's own EQ is set from its own playback screen and overrides this one.",
+  "equalizerActiveOutputLabel": "Active output",
+  "equalizerActiveOutputDefault": "This device's speaker",
+  "equalizerStationsWithOwnEqTitle": "Stations with their own EQ",
+  "equalizerStationsWithOwnEqSubtitle": "Ignore this base equalizer",
+  "equalizerStationsWithOwnEqEmpty": "No stations have their own EQ yet.",
+  "equalizerSaveAsPresetAction": "Save as preset",
+  "equalizerSavePresetDialogTitle": "Save as preset",
+  "equalizerSavePresetNameLabel": "Preset name",
+  "equalizerSavePresetEmptyNameError": "Enter a name for the preset.",
+  "equalizerSavePresetConfirm": "Save",
   "preferredStationTitle": "Preferred station",
   "preferredStationDescription": "Preselected for new alarms and available for quick playback.",
   "preferredStationNoStationsTitle": "No stations available yet",