feat: one-click Discord share and watch links

[vitorhugo-dotnet]

Summary

• Add strict parsing for opaque framerelay://open/share|watch/<token> activations.
• Register the protocol for the current Windows user and forward activations to the existing per-user app instance through a CurrentUserOnly named pipe.
• Consume share links, continue through the normal desktop capture/session flow, then notify RelayControl when the session starts.
• Resolve watch links and join through the existing session runtime path.
• Add API/runtime/parser tests and user-facing integration notes.

Validation

• dotnet build SonicDesktopRelay.sln --no-restore: succeeded, 0 warnings/errors.
• dotnet test SonicDesktopRelay.sln --no-build --no-restore: 386 passed.

Depends on RelayControl API PR: vitorhugo-dotnet/dotnet_RelayControl#58
Bot implementation PR: vitorhugo-dotnet/go_discord_FrameRelay#1
Closes #3