[ibetitsmike]

Merges the coder-desktop-linux Avalonia port back into this repo so one codebase builds and ships Coder Desktop for both Windows and Linux, sharing the VPN service, RPC protocol, and SDKs.

What changed

• Imported from coder-desktop-linux: App.Avalonia (Avalonia UI), App.Shared (UI-agnostic ViewModels/services), App.Linux (Linux service impls), Vpn.Linux (Unix socket RPC transports), Packaging.Linux (deb/rpm/tar/AUR scripts), scripts/run-linux-dev.sh, and the release-linux.yaml workflow.
• Adopted platform-aware versions of shared sources (Vpn.Service/*, MutagenSdk/MutagenClient.cs, Vpn.DebugClient): runtime OS checks plus WINDOWS conditional compilation preserve existing Windows behavior. Only 27 existing files are touched, each with a focused diff.
• Multi-targeted Vpn, Vpn.Service, Tests.Vpn, Tests.Vpn.Service as net8.0;net8.0-windows. ManagerRpc now uses an IRpcServerTransport abstraction: named pipes (extracted to NamedPipeServerTransport) on Windows, Unix domain sockets on Linux.
• Added Coder.Desktop.Linux.slnf (excludes WinUI App, Tests.App, Installer), Directory.Build.props (EnableWindowsTargeting on non-Windows hosts only), and NuGet lock files for the new projects.
• CI: Windows fmt/test/build jobs preserved (fmt passes the solution explicitly since the repo now has both a .sln and a .slnf); added test-linux and build-linux jobs.
• Removed end_of_line = crlf from .editorconfig: the repo is stored with LF line endings and newer dotnet format versions treat the rule as a hard error, which would have forced a whole-repo whitespace rewrite. Existing file bytes are untouched.
• Deflaked SpeakerTest.SendMessageWriteError (shutdown race, ~50% failure rate on Linux).
• The WinUI App and Avalonia App.Avalonia/App.Shared remain separate; converging the Windows app onto App.Shared is follow-up work.

Validation

• All 5 CI checks green: fmt, test-windows, build-windows, test-linux, build-linux.
• Linux (local + fresh dev.coder.com aws-linux workspace): locked-mode restore of Coder.Desktop.Linux.slnf, Release build, all test projects green, format verification clean.
• Linux end-to-end smoke on the workspace: ran CoderVpnService under sudo, drove it via Vpn.DebugClient over the Unix socket. VPN started against https://dev.coder.com, downloaded the tunnel binary, and streamed live workspace/agent peer updates with successful P2P handshakes.
• Windows (dev.coder.com aws-windows-desktop workspace): full-solution restore, dotnet test -p:Platform=x64 on the shared test projects, Release publish of Vpn.Service (net8.0-windows). The WinUI App publish failures there reproduce identically on unmodified main (environment/SDK drift); CI is authoritative and passes.

This PR was prepared by Mux (AI agent) on behalf of Mike.