Song server: central song library with on-demand downloads

- VocaluxeSongServer: ASP.NET Core server that scans an UltraStar library and
  serves the song list (ETag), single files (Range) and zip bundles; optional
  API key; Dockerfile and Gitea workflow for git.disi.dev/projects/vocaluxe-song-server
- Game: config section SongServer (Url, ApiKey). At song loading, the song text
  files are mirrored into a local cache that is loaded like a song folder;
  without connection the last library is used
- VocaluxeLib: CSongFiles hook for all song file access; media are downloaded on
  demand (covers in the background, preview audio, everything before singing
  with progress in the sing screen)
- Tests for scanner, API and downloads (incl. resume); AGENTS.md for the fork

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Output/Languages/en.xml

@@ -267,6 +267,7 @@
   <string name="TR_SCREENSING_BUTTON_RESTART_GAME">Restart game</string>
   <string name="TR_SCREENSING_VOCALSVOLUME">Vocals track volume</string>
   <string name="TR_SCREENSING_NOMORESONGS">No more songs in playlist!</string>
+  <string name="TR_SCREENSING_DOWNLOADING">Downloading %s ... %p%</string>
   <string name="TR_SCREENSING_NOWEBCAM">No webcam available!</string>
   <string name="TR_SCREENSING_ASPECTRATIO">Aspect Ratio: %a</string>
   <string name="TR_SCREENSING_RATING_1">perfect!</string>