fix(ptt,ui): Continuous mode no longer self-disables after 30 s; rename PTT label to Mic

Issue 1: in Continuous transmit mode the talk indicator turned
gray-out / mic disabled after ~30 s and could only be revived by
toggling mic mute. Root cause: SAD-079 MissedKeyUpWatchdog
subscribed to AudioTransmitGate.transmit_active and force-cleared
it after 30 s of true. In PTT mode this is correct (stuck key =
bug). In Continuous mode transmit_active is *supposed* to stay
true indefinitely; the watchdog assumption doesn't hold.

Fix: the watchdog now subscribes to a new ptt_held watch on the
TransmitModeSelector (the raw key-state input, not the resolved
gate). In Continuous mode ptt_held is never set true, so the
watchdog never fires. In PTT mode it still fires on a stuck
key-down as before. The session owns the watchdog (was on the
engine) so it survives engine restarts; it's spawned lazily on the
first start_audio.

MissedKeyUpWatchdog gains spawn_on_signal(rx, on_timeout, timeout)
alongside the existing spawn(gate, timeout) — old shape preserved
for backwards compat. run_watchdog generalised to take any
watch::Receiver<bool> + Box<dyn Fn() + Send + Sync>.

Two new tests:
  - watchdog_on_signal_does_not_fire_when_ptt_held_stays_false
    (the Continuous-mode regression test)
  - watchdog_on_signal_fires_when_signal_stays_true
    (the stuck-key case still fires)

Issue 2: the Voice Bar stats line said 'PTT on/off' even when the
user was in Continuous mode where no PTT key is involved. Renamed
to 'Mic on/off' (mode-neutral) and l10n-ised the on/off literal:
  - en: 'Mic on' / 'Mic off'
  - zh: '麦克风 开启' / '麦克风 关闭'

cargo test --workspace --lib: 80 passed / 0 failed / 1 ignored
(was 78, +2 watchdog tests).
flutter analyze: clean (6 pre-existing Radio.groupValue infos).

apps/chanora_flutter/lib/l10n/app_zh.arb

@@ -66,7 +66,7 @@
   "statusReconnecting": "正在重新连接……第 {attempt} 次尝试，{delay} 秒后",
   "statusConnectionLost": "连接已断开：{reason}",
 
-  "audioStatsLine": "发送 {sent} 帧 • 接收 {received} 帧 • PTT {ptt}",
+  "audioStatsLine": "发送 {sent} 帧 • 接收 {received} 帧 • 麦克风 {ptt}",
 
   "channelsHeading": "频道",
   "clientsHeading": "在线用户",
@@ -83,5 +83,7 @@
   "voiceHardMuteLabel": "麦克风静音",
   "voiceOutputMuteLabel": "扬声器静音",
   "voiceSettingsTitle": "语音设置",
-  "voiceBindKeyAction": "绑定 PTT 按键"
+  "voiceBindKeyAction": "绑定 PTT 按键",
+  "voiceMicOn": "开启",
+  "voiceMicOff": "关闭"
 }