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.
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