Add nixosModules.default with:
- services.jellyfin-lessons.enable toggle
- Configurable dataDir and adminPort options
- Plugin DLL auto-copy to Jellyfin plugins dir
- Admin UI as a user systemd service
The projectFile path was already correct (Jellyfin.Plugin.Lessons.csproj).
nix flake check passes.
- Generated correct deps.json with nuget-to-json for all NuGet dependencies
- Added Jellyfin.Controller and Jellyfin.Model package references with ExcludeAssets=runtime
- Renamed csproj to Jellyfin.Plugin.Lessons.csproj (updated flake.nix projectFile)