fix(audio,storage,ui): allow PTT binding before audio is running

Save-binding before joining a voice channel used to return
BridgeError.invalidCommand(audio not started) because the
PttController only exists after start_audio runs and
set_ptt_binding required a live controller. Users naturally want
to bind their PTT key once on first launch, not every time they
join a channel — fix:

* chanora_storage::IdentityFileStore::set_ptt_binding /
  get_ptt_binding persist the privacy-safe binding triple
  (input_class, platform_key, key_label) into audio_meta.json
  next to transmit_mode and release_tail_ms.
* ChanoraSession holds pending_binding: Arc<Mutex<Option<PttBinding>>>.
  set_ptt_binding now (1) persists to storage best-effort, (2)
  stashes into pending_binding, (3) forwards live to the
  controller only if one exists. No more AudioNotStarted.
* init_storage loads the persisted binding into pending_binding
  so it survives app restarts.
* start_audio applies pending_binding immediately after constructing
  the PttController so the first key-press after join already works.
* supervisor_loop carries pending_binding and re-applies it after
  any reconnect-driven audio engine restart, so reconnects don't
  silently drop the hotkey.
* New bridge call get_ptt_binding() -> (input_class, key_label) plus
  a matching Flutter _hydratePttBinding() in initState lets the
  Voice Bar show the user's saved hotkey label on launch (e.g.
  'PTT: Space') before any voice channel is joined.

cargo test --workspace --lib: 72 passed / 0 failed / 1 ignored.
flutter analyze: clean (6 pre-existing Radio.groupValue infos).
FRB bindings regenerated.

apps/chanora_flutter/lib/main.dart

@@ -145,6 +145,27 @@ class _BetaHomeState extends State<_BetaHome> {
     super.initState();
     _eventsSub = rust.eventsStream().listen(_onEvent);
     unawaited(_reloadBookmarks());
+    unawaited(_hydratePttBinding());
+  }
+
+  /// Hydrate the bound-key display state from the bridge so the
+  /// Voice Bar shows the user's persisted hotkey label immediately
+  /// at launch — without having to wait for them to open the
+  /// binding dialog. The bridge returns empty strings when no
+  /// binding has ever been saved.
+  Future<void> _hydratePttBinding() async {
+    try {
+      final (inputClass, keyLabel) = await rust.getPttBinding();
+      if (!mounted) return;
+      if (keyLabel.isNotEmpty || inputClass.isNotEmpty) {
+        setState(() {
+          _pttBoundKeyLabel = keyLabel;
+          _pttBoundInputClass = inputClass;
+        });
+      }
+    } catch (_) {
+      // No persisted binding or storage not yet wired; not an error.
+    }
   }
 
   Future<void> _reloadBookmarks() async {