feat(ui,voice): add speaker mute button to VoiceBar

Speaker (output) mute existed in the legacy _AudioControls widget
and the rust.setOutputMuted bridge call but was lost when SDD-097
replaced _AudioControls with VoiceBar. Mic mute carried over;
speaker mute did not.

Wire it back: VoiceBar gains an outputMuted prop + onToggleOutputMute
callback and renders a headset/headset_off icon next to the
existing mic mute. main.dart wires the existing _toggleOutputMute
handler (previously dead-code with // ignore: unused_element). The
bridge call setOutputMuted already does both effects together:
local engine silencer + server-broadcast ClientOutputMuted flag.

l10n: rename voiceHardMuteLabel to 'Mute microphone'/'麦克风静音'
to distinguish from the new voiceOutputMuteLabel 'Mute speakers'/
'扬声器静音'.

flutter analyze: clean (6 pre-existing Radio.groupValue infos).

apps/chanora_flutter/lib/l10n/generated/app_localizations.dart

@@ -526,9 +526,15 @@ abstract class AppL10n {
   /// No description provided for @voiceHardMuteLabel.
   ///
   /// In en, this message translates to:
-  /// **'Mute'**
+  /// **'Mute microphone'**
   String get voiceHardMuteLabel;
 
+  /// No description provided for @voiceOutputMuteLabel.
+  ///
+  /// In en, this message translates to:
+  /// **'Mute speakers'**
+  String get voiceOutputMuteLabel;
+
   /// No description provided for @voiceSettingsTitle.
   ///
   /// In en, this message translates to:

apps/chanora_flutter/lib/l10n/generated/app_localizations_en.dart

@@ -255,7 +255,10 @@ class AppL10nEn extends AppL10n {
   String get voiceLeaveAction => 'Leave voice';
 
   @override
-  String get voiceHardMuteLabel => 'Mute';
+  String get voiceHardMuteLabel => 'Mute microphone';
+
+  @override
+  String get voiceOutputMuteLabel => 'Mute speakers';
 
   @override
   String get voiceSettingsTitle => 'Voice settings';

apps/chanora_flutter/lib/l10n/generated/app_localizations_zh.dart

@@ -249,7 +249,10 @@ class AppL10nZh extends AppL10n {
   String get voiceLeaveAction => '离开语音';
 
   @override
-  String get voiceHardMuteLabel => '静音';
+  String get voiceHardMuteLabel => '麦克风静音';
+
+  @override
+  String get voiceOutputMuteLabel => '扬声器静音';
 
   @override
   String get voiceSettingsTitle => '语音设置';