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/widgets/snapshot_view.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 
 import '../l10n/generated/app_localizations.dart';
 import '../services/channel_spacer.dart';
@@ -571,11 +572,40 @@ class _ClientVolumePreference {
 }
 
 class _ClientVolumePreferences extends ChangeNotifier {
-  _ClientVolumePreferences._();
+  _ClientVolumePreferences._() {
+    _load();
+  }
 
   static final instance = _ClientVolumePreferences._();
 
+  static const _prefsKey = 'client_volume_prefs';
+
   final Map<BigInt, _ClientVolumePreference> _byClientId = {};
+  bool _loaded = false;
+
+  Future<void> _load() async {
+    if (_loaded) return;
+    _loaded = true;
+    try {
+      final prefs = await SharedPreferences.getInstance();
+      final raw = prefs.getStringList(_prefsKey) ?? const [];
+      for (final entry in raw) {
+        final parts = entry.split(':');
+        if (parts.length == 3) {
+          final id = BigInt.tryParse(parts[0]);
+          final volume = double.tryParse(parts[1]);
+          final muted = parts[2] == '1';
+          if (id != null && volume != null) {
+            _byClientId[id] = _ClientVolumePreference(
+              volume: volume,
+              muted: muted,
+            );
+          }
+        }
+      }
+      notifyListeners();
+    } catch (_) {}
+  }
 
   _ClientVolumePreference preferenceFor(BigInt clientId) {
     return _byClientId[clientId] ?? const _ClientVolumePreference();
@@ -588,6 +618,17 @@ class _ClientVolumePreferences extends ChangeNotifier {
       _byClientId.remove(clientId);
     }
     notifyListeners();
+    _save();
+  }
+
+  Future<void> _save() async {
+    try {
+      final prefs = await SharedPreferences.getInstance();
+      final raw = _byClientId.entries
+          .map((e) => '${e.key}:${e.value.volume}:${e.value.muted ? 1 : 0}')
+          .toList();
+      await prefs.setStringList(_prefsKey, raw);
+    } catch (_) {}
   }
 }