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/widgets/assistant_message_widget.dart

@@ -18,6 +18,7 @@ import 'package:conduit/l10n/app_localizations.dart';
 import 'enhanced_attachment.dart';
 import 'package:conduit/shared/widgets/chat_action_button.dart';
 import '../../../shared/widgets/model_avatar.dart';
+import '../../../shared/widgets/conduit_components.dart';
 import 'package:url_launcher/url_launcher_string.dart';
 import '../providers/chat_providers.dart' show sendMessageWithContainer;
 import '../../../core/utils/debug_logger.dart';
@@ -457,12 +458,72 @@ class _AssistantMessageWidgetState extends ConsumerState<AssistantMessageWidget>
     }
 
     if (children.isEmpty) return const SizedBox.shrink();
+    // Append TTS karaoke bar if this is the active message
+    final ttsState = ref.watch(textToSpeechControllerProvider);
+    final isActive =
+        ttsState.activeMessageId == _messageId &&
+        (ttsState.status == TtsPlaybackStatus.speaking ||
+            ttsState.status == TtsPlaybackStatus.paused ||
+            ttsState.status == TtsPlaybackStatus.loading);
+    if (isActive && ttsState.activeSentenceIndex >= 0) {
+      children.add(const SizedBox(height: Spacing.sm));
+      children.add(_buildKaraokeBar(ttsState));
+    }
+
     return Column(
       crossAxisAlignment: CrossAxisAlignment.start,
       children: children,
     );
   }
 
+  Widget _buildKaraokeBar(TextToSpeechState ttsState) {
+    final theme = context.conduitTheme;
+    final idx = ttsState.activeSentenceIndex;
+    if (idx < 0 || idx >= ttsState.sentences.length) {
+      return const SizedBox.shrink();
+    }
+    final sentence = ttsState.sentences[idx];
+    final ws = ttsState.wordStartInSentence;
+    final we = ttsState.wordEndInSentence;
+
+    final baseStyle = TextStyle(
+      color: theme.textPrimary,
+      height: 1.2,
+      fontSize: 14,
+    );
+    final highlightStyle = baseStyle.copyWith(
+      backgroundColor: theme.buttonPrimary.withValues(alpha: 0.25),
+      color: theme.textPrimary,
+      fontWeight: FontWeight.w600,
+    );
+
+    InlineSpan buildSpans() {
+      if (ws == null ||
+          we == null ||
+          ws < 0 ||
+          we <= ws ||
+          ws >= sentence.length) {
+        return TextSpan(text: sentence, style: baseStyle);
+      }
+      final safeEnd = we.clamp(0, sentence.length);
+      final before = sentence.substring(0, ws);
+      final word = sentence.substring(ws, safeEnd);
+      final after = sentence.substring(safeEnd);
+      return TextSpan(
+        children: [
+          if (before.isNotEmpty) TextSpan(text: before, style: baseStyle),
+          TextSpan(text: word, style: highlightStyle),
+          if (after.isNotEmpty) TextSpan(text: after, style: baseStyle),
+        ],
+      );
+    }
+
+    return ConduitCard(
+      padding: const EdgeInsets.all(Spacing.sm),
+      child: RichText(text: buildSpans()),
+    );
+  }
+
   bool get _shouldShowTypingIndicator =>
       widget.isStreaming && _isAssistantResponseEmpty;