feat(ui,ptt): on-screen touch-and-hold PTT button for iOS / iPadOS / Android

iOS / iPadOS / Android have no hardware keyboard for the user to
bind a PTT key on. Up to now the VoiceBar showed only a
'Push to talk: bound key —' hint that didn't lead anywhere usable.

Add a touch-and-hold on-screen PTT button rendered only on
touch-only platforms (Platform.isIOS || Platform.isAndroid; web
hosts and desktop continue to use the hardware-key path
unchanged).

apps/chanora_flutter/lib/widgets/voice_bar.dart:
  * New module-private `_isTouchOnlyPttHost` predicate.
  * VoiceBar gains an `onPttHeldChanged: ValueChanged<bool>`
    constructor param. Desktop callers wire it but never invoke it
    because the button is not rendered there.
  * Row 3 (the PTT-only secondary content) now branches:
    - on touch-only hosts -> renders the new `_PttHoldButton` plus
      a small release-tail hint underneath
    - on hardware-keyboard hosts -> renders the same bound-key +
      release-tail one-liner as before, unchanged.
  * New `_PttHoldButton` StatefulWidget. Uses a single
    GestureDetector covering onTapDown / onTapUp / onTapCancel /
    onPanDown / onPanEnd / onPanCancel so the held edges fire on
    finger-down and the released edge fires when the user lifts
    OR drags off OR another gesture in the arena wins. Visual
    feedback mirrors the level-meter active flag.

apps/chanora_flutter/lib/main.dart:
  * New `_onOnscreenPttHeldChanged(bool held)` method that calls
    `rust.setPtt(active: held)`. The bridge's set_ptt routes the
    edge through the same release-tail timer + transmit-mode
    selector that desktop hardware keys use (SDD-096 / SAD-083),
    so behaviour parity is preserved.

flutter analyze: clean (6 pre-existing Radio.groupValue infos).
flutter build ios --release --no-codesign: clean (Runner.app 29.9 MB).

DEC-025: iPhone + iPad + Android in scope; this commit makes PTT
mode actually usable on those platforms. The 'Focused' capability
badge wording in ios-p0-acceptance.md / ipad-p0-acceptance.md
already documents the on-screen button as the only PTT input;
this commit makes that documentation true.

apps/chanora_flutter/lib/main.dart

@@ -461,6 +461,31 @@ class _BetaHomeState extends State<_BetaHome> {
     }
   }
 
+  /// Touch-only PTT (iOS / iPadOS / Android). The on-screen
+  /// `_PttHoldButton` calls this with `true` on finger-down and
+  /// `false` on finger-up (or cancel). The bridge's
+  /// `setPtt(active:)` routes the press edge through the same
+  /// release-tail timer + transmit-mode selector the desktop
+  /// hardware-key paths use (SDD-096 / SAD-083), so the user-
+  /// visible behaviour is identical across platforms — only the
+  /// input device changes.
+  ///
+  /// Errors are swallowed silently in the held=false branch
+  /// because the timer's `key_up` is idempotent; a failed send
+  /// would still let the tail expire naturally. Errors on
+  /// held=true surface in the UI banner so the user knows the
+  /// mic didn't open.
+  Future<void> _onOnscreenPttHeldChanged(bool held) async {
+    try {
+      await rust.setPtt(active: held);
+    } catch (e) {
+      if (held) {
+        if (!mounted) return;
+        setState(() => _error = e.toString());
+      }
+    }
+  }
+
   Future<void> _onOpenVoiceSettings() async {
     final result = await showDialog<VoiceSettingsResult>(
       context: context,
@@ -943,6 +968,7 @@ class _BetaHomeState extends State<_BetaHome> {
                       onToggleMute: _onToggleHardMute,
                       onToggleOutputMute: _toggleOutputMute,
                       onConfigure: _onOpenVoiceSettings,
+                      onPttHeldChanged: _onOnscreenPttHeldChanged,
                     );
                     final snapshotView = _SnapshotView(
                       snapshot: _snapshot!,