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/app_en.arb

@@ -123,7 +123,8 @@
   "voiceReleaseTailLabel": "Release tail",
   "voiceReleaseTailHint": "ms",
   "voiceLeaveAction": "Leave voice",
-  "voiceHardMuteLabel": "Mute",
+  "voiceHardMuteLabel": "Mute microphone",
+  "voiceOutputMuteLabel": "Mute speakers",
   "voiceSettingsTitle": "Voice settings",
   "voiceBindKeyAction": "Bind PTT key"
 }