Add Avalonia GUI and Core library

[matziq]

Summary

• Adds a new Avalonia-based GUI project (RecordingDownload.Gui.csproj) for Ring Recording Download.
• Introduces a shared Core library (RecordingDownload.Core.csproj) to reuse logic across CLI + GUI.
• Updates KoenZomers.Ring.RecordingDownload.sln and adds a ProjectReference from ConsoleAppCore to Core.

What changed

• New projects: Gui, Core
• Solution updated to include RecordingDownload.Gui and RecordingDownload.Core
• ConsoleAppCore.csproj now references the Core project

Notes

• No bin/ or obj/ outputs are included—source only.
• Built locally with .NET 8 SDK.

Testing

• dotnet build ConsoleAppCore/ConsoleAppCore.csproj
• (Optional) dotnet build Gui/RecordingDownload.Gui.csproj