Hide transcript header timestamps

[ComputelessComputer]

Remove visible transcript segment time ranges while preserving word timing data for playback and highlighting.

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Note

Low Risk
Presentation-only change in the transcript note-input UI with no auth, data, or playback logic touched.

Overview
Transcript segment headers no longer show time ranges (e.g. 00:12 - 00:18). The sticky header now only renders the speaker assignment control; word-level start_ms / end_ms on segments and words are unchanged for playback and highlighting elsewhere.

Unused formatTimestamp and getTimestampRange helpers were removed from the transcript renderer utils module, and the SegmentHeader test was updated to assert timestamps are not in the DOM.

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