The backup envelope carried favorites, EQ presets, alarms and the
multi-device toggle but not EstadoEcualizador's own on/off flag, so
restoring a backup on another device silently kept that device's
existing toggle state instead of the source device's.
Bumps the backup schema to v4 (additive over v3): the flag is only
written when explicitly provided, so old exports stay at v2/v3.
Importing an old backup without the field leaves the current toggle
untouched rather than defaulting it. Applying the imported value
reuses EstadoEcualizador.cambiarActivo so it persists and pushes to
the live audio engine exactly like a manual toggle.
Introduce multi-device EQ support allowing each audio output device
(built-in speaker, wired, USB, individual Bluetooth by MAC) to have
its own equalizer preset, combined with existing per-station presets
for a full station×device matrix.
- Add DispositivoAudio model and ServicioDispositivoAudio interface
- Add Android platform channel (AudioDeviceCallback) for device detection
- Add iOS AudioDevicesPlugin (AVAudioSession route tracking)
- Extend ServicioEcualizador with device and matrix persistence keys
- Implement 4-level resolution: matrix > station > device > global
- Add advanced EQ settings section with feature toggle (off by default)
- Extend export/import to v3 with backward compatibility
- 184 tests passing, zero analyzer issues
- New ServicioExportImport owns the v2 backup envelope, pretty JSON encode and graceful decode; byte-compatible with existing exports, locked by a round-trip test
- pantalla_ajustes delegates backup serialization to the service (inline jsonDecode/jsonEncode removed)
- New EstadoEcualizador ChangeNotifier owns all EQ state and persistence (principal/current/per-station presets, active flag), exposed via its own provider so EQ changes no longer rebuild EstadoRadio consumers
- EstadoRadio slims down ~210 lines and keeps 15 delegating compat members marked TODO(S4b) for the next slice to remove
- Player EQ toggle rewired to the new provider to avoid going stale
- 4 new tests (103 total green), flutter analyze clean
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