feat(flutter): persist voice settings and per-user volume (TODO-039,040)

Add SharedPreferences persistence for transmit mode, release tail,
input/output device. Per-user volume/mute persisted across sessions.

apps/chanora_flutter/test/services/ui_preferences_service_test.dart

@@ -67,4 +67,44 @@ void main() {
 
     expect(settings.themeMode, UiThemeMode.system);
   });
+
+  test('loads null voice settings when unset', () async {
+    final settings = await service.loadSettings();
+
+    expect(settings.transmitModeIndex, isNull);
+    expect(settings.releaseTailMs, isNull);
+    expect(settings.inputDeviceId, isNull);
+    expect(settings.outputDeviceId, isNull);
+  });
+
+  test('saves and loads transmit mode index', () async {
+    await service.saveTransmitModeIndex(1);
+    final settings = await service.loadSettings();
+
+    expect(settings.transmitModeIndex, 1);
+  });
+
+  test('saves and loads release tail ms', () async {
+    await service.saveReleaseTailMs(300);
+    final settings = await service.loadSettings();
+
+    expect(settings.releaseTailMs, 300);
+  });
+
+  test('saves and loads audio device ids', () async {
+    await service.saveInputDeviceId('input-123');
+    await service.saveOutputDeviceId('output-456');
+    final settings = await service.loadSettings();
+
+    expect(settings.inputDeviceId, 'input-123');
+    expect(settings.outputDeviceId, 'output-456');
+  });
+
+  test('removes audio device id when set to null', () async {
+    await service.saveInputDeviceId('input-123');
+    await service.saveInputDeviceId(null);
+    final settings = await service.loadSettings();
+
+    expect(settings.inputDeviceId, isNull);
+  });
 }