feat(tts): add karaoke-style TTS progress bar to assistant UI

Add rendering and support for a karaoke-style text-to-speechprogress bar in assistant messages so users can see the currently
spoken sentence and highlighted word during playback.

- Append TTS karaoke bar to AssistantMessageWidget when the message is
  the active TTS target and playback is speaking/paused/loading.
- Implement _buildKaraokeBar to render the active sentence with a
  highlighted word span, using ConduitCard and theme styles.
- Import conduit_components for shared UI primitives.
- Extend TextToSpeechState with sentence data:
  sentences, sentenceOffsets, activeSentenceIndex, and per-word
  progress (wordStartInSentence, wordEndInSentence).
- Add provider callbacks wiring: onSentenceIndex and
  onDeviceWordProgress handlers (hooked into TTS backend).
- Prepare sentence splitting and word-progress plumbing in the TTS
  provider (prepares data used to drive the karaoke display).

This change improves UX by visually indicating the spoken sentence
and current word during TTS playback, aiding comprehension and
accessibility.

lib/features/chat/services/text_to_speech_service.dart

@@ -31,6 +31,8 @@ class TextToSpeechService {
   VoidCallback? _onPause;
   VoidCallback? _onContinue;
   void Function(String message)? _onError;
+  void Function(int sentenceIndex)? _onSentenceIndex;
+  void Function(int start, int end)? _onDeviceWordProgress;
 
   bool get isInitialized => _initialized;
   bool get isAvailable => _available;
@@ -51,6 +53,8 @@ class TextToSpeechService {
     VoidCallback? onPause,
     VoidCallback? onContinue,
     void Function(String message)? onError,
+    void Function(int sentenceIndex)? onSentenceIndex,
+    void Function(int start, int end)? onDeviceWordProgress,
   }) {
     _onStart = onStart;
     _onComplete = onComplete;
@@ -58,6 +62,8 @@ class TextToSpeechService {
     _onPause = onPause;
     _onContinue = onContinue;
     _onError = onError;
+    _onSentenceIndex = onSentenceIndex;
+    _onDeviceWordProgress = onDeviceWordProgress;
 
     _tts.setStartHandler(_handleStart);
     _tts.setCompletionHandler(_handleComplete);
@@ -65,6 +71,13 @@ class TextToSpeechService {
     _tts.setPauseHandler(_handlePause);
     _tts.setContinueHandler(_handleContinue);
     _tts.setErrorHandler(_handleError);
+    try {
+      _tts.setProgressHandler((String text, int start, int end, String word) {
+        _onDeviceWordProgress?.call(start, end);
+      });
+    } catch (_) {
+      // Some platforms may not support progress handler
+    }
   }
 
   /// Initialize the native TTS engine lazily
@@ -151,6 +164,7 @@ class TextToSpeechService {
     if (result is int && result != 1) {
       _onError?.call('Text-to-speech engine returned code $result');
     }
+    _onSentenceIndex?.call(0);
   }
 
   Future<void> pause() async {
@@ -370,6 +384,7 @@ class TextToSpeechService {
     _buffered.add(Uint8List.fromList(firstBytes));
     _currentIndex = 0;
     await _player.play(BytesSource(_buffered.first));
+    _onSentenceIndex?.call(0);
 
     // Prefetch the rest in background
     unawaited(
@@ -438,6 +453,7 @@ class TextToSpeechService {
     _currentIndex = nextIndex;
     final bytes = _buffered[nextIndex];
     await _player.play(BytesSource(bytes));
+    _onSentenceIndex?.call(_currentIndex);
   }
 
   List<String> _splitForTts(String text) {