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/widgets/voice_bar.dart

@@ -18,6 +18,7 @@ class VoiceBar extends StatelessWidget {
     required this.inChannel,
     required this.transmitMode,
     required this.hardMute,
+    required this.outputMuted,
     required this.releaseTailMs,
     required this.channelName,
     required this.audioStats,
@@ -26,6 +27,7 @@ class VoiceBar extends StatelessWidget {
     required this.pttBoundInputClass,
     required this.pttBoundKeyLabel,
     required this.onToggleMute,
+    required this.onToggleOutputMute,
     required this.onConfigure,
     required this.onLeave,
   });
@@ -39,6 +41,13 @@ class VoiceBar extends StatelessWidget {
   /// Hard-mute clamp state.
   final bool hardMute;
 
+  /// Speaker (output) mute state. Mirrors the server-broadcast
+  /// `ClientOutputMuted` flag plus the engine's local output
+  /// silencer — toggling this hushes incoming voice immediately
+  /// AND tells the server so other clients see the headphone-off
+  /// icon next to our name.
+  final bool outputMuted;
+
   /// Configured release-tail in milliseconds (0..=500). Surfaced as
   /// a hint underneath the mode badge.
   final int releaseTailMs;
@@ -67,6 +76,9 @@ class VoiceBar extends StatelessWidget {
   /// Toggle the hard-mute clamp.
   final VoidCallback onToggleMute;
 
+  /// Toggle speaker (output) mute.
+  final VoidCallback onToggleOutputMute;
+
   /// Open the voice settings dialog. This is the SINGLE entry point
   /// for transmit-mode selection, PTT key binding, and release-tail
   /// configuration. The capability badge below is information-only
@@ -136,6 +148,15 @@ class VoiceBar extends StatelessWidget {
                   ),
                 ],
                 const Spacer(),
+                IconButton(
+                  tooltip: l10n.voiceOutputMuteLabel,
+                  icon: Icon(
+                    outputMuted ? Icons.headset_off : Icons.headset,
+                  ),
+                  isSelected: outputMuted,
+                  selectedIcon: const Icon(Icons.headset_off),
+                  onPressed: onToggleOutputMute,
+                ),
                 IconButton(
                   tooltip: l10n.voiceHardMuteLabel,
                   icon: Icon(hardMute ? Icons.mic_off : Icons.mic),