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/generated/app_localizations.dart

@@ -454,7 +454,7 @@ abstract class AppL10n {
   /// No description provided for @audioStatsLine.
   ///
   /// In en, this message translates to:
-  /// **'TX {sent} frames • RX {received} frames • PTT {ptt}'**
+  /// **'TX {sent} frames • RX {received} frames • Mic {ptt}'**
   String audioStatsLine(int sent, int received, String ptt);
 
   /// No description provided for @channelsHeading.
@@ -546,6 +546,18 @@ abstract class AppL10n {
   /// In en, this message translates to:
   /// **'Bind PTT key'**
   String get voiceBindKeyAction;
+
+  /// No description provided for @voiceMicOn.
+  ///
+  /// In en, this message translates to:
+  /// **'on'**
+  String get voiceMicOn;
+
+  /// No description provided for @voiceMicOff.
+  ///
+  /// In en, this message translates to:
+  /// **'off'**
+  String get voiceMicOff;
 }
 
 class _AppL10nDelegate extends LocalizationsDelegate<AppL10n> {

apps/chanora_flutter/lib/l10n/generated/app_localizations_en.dart

@@ -216,7 +216,7 @@ class AppL10nEn extends AppL10n {
 
   @override
   String audioStatsLine(int sent, int received, String ptt) {
-    return 'TX $sent frames • RX $received frames • PTT $ptt';
+    return 'TX $sent frames • RX $received frames • Mic $ptt';
   }
 
   @override
@@ -265,4 +265,10 @@ class AppL10nEn extends AppL10n {
 
   @override
   String get voiceBindKeyAction => 'Bind PTT key';
+
+  @override
+  String get voiceMicOn => 'on';
+
+  @override
+  String get voiceMicOff => 'off';
 }

apps/chanora_flutter/lib/l10n/generated/app_localizations_zh.dart

@@ -210,7 +210,7 @@ class AppL10nZh extends AppL10n {
 
   @override
   String audioStatsLine(int sent, int received, String ptt) {
-    return '发送 $sent 帧 • 接收 $received 帧 • PTT $ptt';
+    return '发送 $sent 帧 • 接收 $received 帧 • 麦克风 $ptt';
   }
 
   @override
@@ -259,4 +259,10 @@ class AppL10nZh extends AppL10n {
 
   @override
   String get voiceBindKeyAction => '绑定 PTT 按键';
+
+  @override
+  String get voiceMicOn => '开启';
+
+  @override
+  String get voiceMicOff => '关闭';
 }