- 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
