Add Jellyfin streaming service support

[rkdx7]

Summary

This PR adds support for Jellyfin as a streaming service, so EasySubs
can be used on self-hosted Jellyfin media servers.

What's included

• New Jellyfin service (src/streamings/jellyfin.ts) implementing the
  Service interface: subtitle fetching, subs container, settings
  containers and live-stream detection.
• Automatic detection in getCurrentService() using the Jellyfin
  application-name meta tag, the global ApiClient, and a DOM fallback.
• Subtitle track selector (JellyfinSubTrack component) shown in the
  settings panel only when the active service is Jellyfin.
• Reload-free injection: the popup now injects the content script via
  chrome.scripting instead of reloading the tab, so SPA players (like
  Jellyfin) don't lose their playback state. Falls back to a reload if
  injection fails.
• Service detection retries when the page isn't ready yet, and a
  no-op ServiceStub.init() instead of throwing.
• UI fixes: stop click propagation inside the subtitles and settings
  containers so player controls aren't toggled, plus styling fixes for
  subtitles and the progress bar.
• Generalized the popup action label from "Enable on Kinopub" to
  "Enable on this site".

Testing

Tested manually on a Jellyfin web player: service is detected, subtitles
render, the subtitle-track selector works, and translations behave as
expected.