feat(eq): add per-device equalizer with 4-level preset resolution

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

lib/l10n/app_en.arb

@@ -616,5 +616,16 @@
   "stationCount": "{count, plural, =1{1 station} other{{count} stations}}",
   "alarmIconLabel": "Musical alarm",
   "vacationIconLabel": "Vacation mode",
-  "streamUrlHint": "https://stream.example.com:8000/radio"
+  "streamUrlHint": "https://stream.example.com:8000/radio",
+  "advancedEqSectionTitle": "Advanced Equalization Options",
+  "advancedEqEnableToggle": "Enable per-device EQ",
+  "advancedEqEnableToggleSubtitle": "Automatically apply a different EQ preset when your audio output changes.",
+  "advancedEqKnownDevicesTitle": "Known audio devices",
+  "advancedEqKnownDevicesEmpty": "No audio devices recorded yet. Connect a device to get started.",
+  "advancedEqDevicePresetLabel": "Preset: {presetName}",
+  "@advancedEqDevicePresetLabel": {
+    "placeholders": {
+      "presetName": {}
+    }
+  }
 }