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/main.dart

@@ -318,7 +318,6 @@ class _BetaHomeState extends State<_BetaHome> {
     }
   }
 
-  // ignore: unused_element
   Future<void> _toggleOutputMute() async {
     final next = !_outputMuted;
     try {
@@ -846,6 +845,7 @@ class _BetaHomeState extends State<_BetaHome> {
                 inChannel: _inChannel,
                 transmitMode: _transmitMode,
                 hardMute: _hardMute,
+                outputMuted: _outputMuted,
                 releaseTailMs: _releaseTailMs,
                 channelName: _currentVoiceChannelName(),
                 audioStats: _audioStats,
@@ -854,6 +854,7 @@ class _BetaHomeState extends State<_BetaHome> {
                 pttBoundInputClass: _pttBoundInputClass,
                 pttBoundKeyLabel: _pttBoundKeyLabel,
                 onToggleMute: _onToggleHardMute,
+                onToggleOutputMute: _toggleOutputMute,
                 onConfigure: _onOpenVoiceSettings,
                 onLeave: _onLeaveVoice,
               ),