Fix polling after seek during playback

[happykawayigt]

Summary

• reschedule transcript polling after a seek while playback is still running
• preserve paused-seek behavior without starting a polling loop
• add a regression test for the stale-timer scenario

Root cause

handleSeeked() updated the visual state but left the timer scheduled from the pre-seek position. Seeking from a long silence into dense speech could therefore leave highlighting frozen until that stale timer fired.

Validation

• npm run build
• npm test -- --runInBand (72 tests passed)

Fixes #264