fix(media): rewind instead of reloading once the media has played

[skjnldsv]

Replaying a video downloaded the whole file again, every time.

Fixes nextcloud/viewer#2585

donePlaying() reloaded the element (media.load()) once it ended, to bring the poster back. A reload throws away what was buffered, and Firefox fetched the file again on the next play. It now stops the player instead: paused at the start, which is what plyr shows its poster over (.plyr--stopped.plyr__poster-enabled), and the buffer stays.

The poster after playing is a feature people use, a film and a picture of the same name side by side, so it gets tests. The playground has trailer.webm with trailer.jpg beside it, and an e2e test plays it to the end, checks the poster is back on top, plays it again and checks nothing was fetched. The video is served no-store there, otherwise Chromium answers the reload from its cache and the test passes on main too; with it, main makes a fourth request and fails. Unit tests cover stop() being used rather than load(), and a rewind by hand when plyr has no player yet.

Based on #86, without which the video never gets its poster in the first place. Only run locally in Chromium; Firefox runs in CI.

👾 This pull request was assisted by Claude Code, commits carry an Assisted-by trailer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.56%. Comparing base (3d47c85) to head (3286477).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
+ Coverage   89.79%   90.56%   +0.76%     
==========================================
  Files          39       39              
  Lines        2969     2978       +9     
  Branches      642      649       +7     
==========================================
+ Hits         2666     2697      +31     
+ Misses        293      270      -23     
- Partials       10       11       +1     

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