BADGER-16: hide subtitles when seeking away

[JoeBBC]

📺 What

Fixes a bug where IMSC subtitles remained on-screen after seeking to a time without a subtitle

🛠 How

After a seek is detected:

• Removes the subtitle DOM element
• Clears state to ensure a check is always performed to render a subtitle if available

A seek is detected if, over a 750ms period, playback time reverses, or jumps forward by > 2 seconds

✅ Testing
Test Guidelines

Test engineer sign off	❌

Added 3 unit tests which operate with mock data:

• Ensure subtitles are removed after seeking back to a time without a subtitle
• Ensure subtitles remain rendered after seeking forward to a time with a subtitle (within the same long-duration subtitle)
• Ensure subtitles remain rendered after seeking back & forth (within the same long-duration subtitle) between different segments of a live-stream