Song server: search and download songs, mobile web UI

- Sources: "free" (Creative Commons songs with media from
  github.com/UltraStar-Deluxe/songs) and "usdb" (usdb.animux.de with an
  account; audio/video via yt-dlp from the song's #VIDEO hints or YouTube
  links in the comments, covers from fanart.tv/YouTube)
- Aggregated search (all sources in parallel, in-library flag), download
  queue into <library>/Downloads/<source>/<Artist - Title>, then rescan
- Web UI in wwwroot (plain JS, no build): search, downloads with progress,
  library; phone first, dark mode, installable
- Docker image with yt-dlp, ffmpeg and deno; library mounted writable
- Tests for matching, USDB parsing and song rewriting

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

AGENTS.md

@@ -8,15 +8,15 @@ hook-based, so upstream can still be merged.
 1. **Song server** (`VocaluxeSongServer/`, in progress): a central song library on the homelab. The game becomes the
    frontend and downloads song files on demand. See `VocaluxeSongServer/README.md`.
 2. Later: serve the guest WebApp ("browse by phone", `WebApp/` + `VocaluxeWeb/`) directly from the server.
-3. Later: search and download songs that aren't in the library, from common internet sources (possibly UltraSinger
-   to create songs from audio).
+3. Search and download songs that aren't in the library (started: server web UI with the sources `free` and `usdb`;
+   next: more sources, UltraSinger to create songs from audio).
 
 ## Layout
 | Folder | What |
 |---|---|
 | `Vocaluxe/` | The game. Song server client: `Base/SongServer/` (`CSongServer` sync, `CRemoteSongFileProvider` downloads) |
 | `VocaluxeLib/` | Shared library. `Songs/CSongFiles.cs` is the hook every song file access goes through |
-| `VocaluxeSongServer/` | ASP.NET Core server, Docker image `git.disi.dev/projects/vocaluxe-song-server` |
+| `VocaluxeSongServer/` | ASP.NET Core server, Docker image `git.disi.dev/projects/vocaluxe-song-server`. `Sources/`: song search and downloads (free CC songs from GitHub, USDB + yt-dlp). `wwwroot/`: mobile web UI (plain JS, no build) |
 | `VocaluxeWeb/`, `WebApp/` | Built-in webserver of the game and the guest WebApp (Vue), upstream |
 | `Tests/` | NUnit; song server tests in `Tests/SongServer` |
 
@@ -32,6 +32,12 @@ hook-based, so upstream can still be merged.
 - Config: `config.xml` section `SongServer` (`Url`, `ApiKey`). The contracts (`VocaluxeSongServer/Contracts`) are
   linked into `Vocaluxe.csproj`; the test project references the server with `extern alias songserver`.
 
+## Song server web UI and sources
+- The web UI is plain HTML/CSS/JS in `VocaluxeSongServer/wwwroot` (no build step, no external requests except cover
+  images). Check it at phone width: headless Edge has a ~500px minimum window, so render the page in a 390px iframe.
+- USDB can't be tested without an account (search and text download need a login). Its HTML parsing is covered by
+  `Tests/SongServer/CSongSourcesTest.cs`; if USDB changes its HTML, fix `CUsdbSongSource.ParseList` there.
+
 ## Build and test (Windows)
 - `dotnet build Vocaluxe/Vocaluxe.csproj -c DebugWin -p:Platform=x64` (the whole solution also builds the native
   `PitchTracker`, which needs Visual Studio's C++ tools).