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Add a "Highlight Current Sentence" keyboard action (default Shift+M, in the Text to Speech shortcut section) that persists the sentence TTS is reading aloud as a normal highlight using the user's default style/color — no text selection, eyes-off, silent, and idempotent (a repeat press on the same sentence is a no-op rather than a duplicate). Flow: the shortcut handler in useBookShortcuts dispatches tts-highlight-sentence → useTTSControl (which owns the TTSController) resolves the current sentence via the new TTSController.getSpokenSentence() and relays create-tts-highlight → Annotator builds the BookNote with the pure, unit-tested buildTTSSentenceHighlight helper and persists/renders it like any other highlight. Closes #4085 Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>