Convert SRT subtitles to WebVTT and handle cue formatting differences

Verified steps

1. Add the required 'WEBVTT' header line at the very top of the file — SRT files have no header and start directly with the first cue index
2. Replace every comma millisecond separator in timestamps with a period: change '00:01:12,500 --> 00:01:15,000' to '00:01:12.500 --> 00:01:15.000' — this is the only timing change needed
3. Decide whether to keep cue sequence numbers: WebVTT makes them optional, so you may strip them or replace them with meaningful cue identifiers (e.g., 'intro-1') for JavaScript cue events
4. Add WebVTT cue settings if positioning is needed — append settings after the timestamp arrow on the same line, such as 'align:center position:50% size:80%'; SRT has no equivalent
5. Validate the output with a WebVTT parser or the W3C Nu validator — common failures include missing WEBVTT header, BOM encoding issues, and stray Windows line endings (CRLF inside cue text)
6. Serve WebVTT files with Content-Type 'text/vtt' and ensure CORS headers allow cross-origin fetch if the file is on a different domain from the video player