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>

Tests/VocaluxeLib/XML/TestFiles/SConfig.xml

@@ -137,4 +137,8 @@
     <ServerPort>3000</ServerPort>
     <SongCountCoverThreshold>70</SongCountCoverThreshold>
   </Server>
+  <SongServer>
+    <Url />
+    <ApiKey />
+  </SongServer>
 </root>