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_en.arb

@@ -96,7 +96,7 @@
     "placeholders": { "reason": { "type": "String" } }
   },
 
-  "audioStatsLine": "TX {sent} frames • RX {received} frames • PTT {ptt}",
+  "audioStatsLine": "TX {sent} frames • RX {received} frames • Mic {ptt}",
   "@audioStatsLine": {
     "placeholders": {
       "sent": { "type": "int" },
@@ -126,5 +126,7 @@
   "voiceHardMuteLabel": "Mute microphone",
   "voiceOutputMuteLabel": "Mute speakers",
   "voiceSettingsTitle": "Voice settings",
-  "voiceBindKeyAction": "Bind PTT key"
+  "voiceBindKeyAction": "Bind PTT key",
+  "voiceMicOn": "on",
+  "voiceMicOff": "off"
 }