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/lib/services/ui_preferences_service.dart

@@ -18,11 +18,19 @@ class UiSettings {
     this.host = '',
     this.nickname = '',
     this.themeMode = UiThemeMode.system,
+    this.transmitModeIndex,
+    this.releaseTailMs,
+    this.inputDeviceId,
+    this.outputDeviceId,
   });
 
   final String host;
   final String nickname;
   final UiThemeMode themeMode;
+  final int? transmitModeIndex;
+  final int? releaseTailMs;
+  final String? inputDeviceId;
+  final String? outputDeviceId;
 }
 
 class UiPreferencesService {
@@ -30,6 +38,10 @@ class UiPreferencesService {
   static const _nicknameKey = 'ui.nickname';
   static const _themeModeKey = 'ui.theme_mode';
   static const _permissionsExplainedKey = 'perms_explained';
+  static const _transmitModeIndexKey = 'voice.transmit_mode_index';
+  static const _releaseTailMsKey = 'voice.release_tail_ms';
+  static const _inputDeviceIdKey = 'audio.input_device_id';
+  static const _outputDeviceIdKey = 'audio.output_device_id';
 
   const UiPreferencesService();
 
@@ -39,6 +51,10 @@ class UiPreferencesService {
       host: prefs.getString(_hostKey) ?? '',
       nickname: prefs.getString(_nicknameKey) ?? '',
       themeMode: UiThemeMode.fromStorage(prefs.getString(_themeModeKey)),
+      transmitModeIndex: prefs.getInt(_transmitModeIndexKey),
+      releaseTailMs: prefs.getInt(_releaseTailMsKey),
+      inputDeviceId: prefs.getString(_inputDeviceIdKey),
+      outputDeviceId: prefs.getString(_outputDeviceIdKey),
     );
   }
 
@@ -53,6 +69,34 @@ class UiPreferencesService {
     await prefs.setString(_themeModeKey, themeMode.name);
   }
 
+  Future<void> saveTransmitModeIndex(int index) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setInt(_transmitModeIndexKey, index);
+  }
+
+  Future<void> saveReleaseTailMs(int ms) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setInt(_releaseTailMsKey, ms);
+  }
+
+  Future<void> saveInputDeviceId(String? id) async {
+    final prefs = await SharedPreferences.getInstance();
+    if (id == null) {
+      await prefs.remove(_inputDeviceIdKey);
+    } else {
+      await prefs.setString(_inputDeviceIdKey, id);
+    }
+  }
+
+  Future<void> saveOutputDeviceId(String? id) async {
+    final prefs = await SharedPreferences.getInstance();
+    if (id == null) {
+      await prefs.remove(_outputDeviceIdKey);
+    } else {
+      await prefs.setString(_outputDeviceIdKey, id);
+    }
+  }
+
   Future<bool> hasExplainedPermissions() async {
     final prefs = await SharedPreferences.getInstance();
     return prefs.getBool(_permissionsExplainedKey) ?? false;