Add LoginFix, fixes the Canva sign-in crash under Wine

[Gamedirection]

Summary

Adds LoginFix, a plugin that fixes the Canva sign-in flow crashing under Wine. WineFix's own README already lists this as a known limitation:

Canva sign-in dialog patched out (temporary; pending protocol handler fix)

LoginFix is that fix. It lets sign-in complete instead of disabling it.

Root cause

Serif.Affinity.Application.ProcessCommandLineArguments references a WinRT type Wine does not implement (SharedStorageAccessManager), only reached on an unrelated affinity-open-file: argument. The .NET CLR resolves every type in a method body at JIT time. So any call into that method throws TypeLoadException, including the affinity:// OAuth callback path the sign-in flow depends on.

Harmony cannot patch ProcessCommandLineArguments directly either. Patching a method, even with a plain prefix, requires Harmony to decompile its IL. That means resolving every operand in the body, including the poisoned call.

The fix

LoginFix patches the method's two callers instead:

• ProcessArguments(), the app's own startup command line
• SingleInstanceThread(), the named-pipe listener that receives the sign-in callback from a second launched instance

Both only reference the poisoned type by signature, which Harmony can resolve safely. Both are fully replaced with a safe reimplementation that never calls the real ProcessCommandLineArguments. The only thing dropped is the unrelated affinity-open-file: handling, which needs SharedStorageAccessManager and cannot work under Wine regardless.

Structure

Follows WineFix's exact pattern:

• LoginFix/LoginFix.csproj, LoginFixPlugin.cs, Patches/ProcessCommandLineArgumentsPatch.cs
• LoginFix/README.md, LoginFix/LICENSE (GPLv2, with the same Canva Zero-Clause BSD exemption WineFix uses)
• Registered in AffinityPluginLoader.sln
• docs/loginfix/index.md, linked from docs/index.md and mkdocs.yml

Compatibility with WineFix

LoginFix and WineFix patch different methods and can run together. If both are installed, WineFix's sign-in dialog suppression and LoginFix's working sign-in flow may conflict, since one hides the dialog and the other completes it. Noted in both READMEs. Whether to adjust WineFix's own patch once LoginFix exists is your call, I have not touched WineFix's code in this PR.

Test plan

• dotnet build LoginFix/LoginFix.csproj -c Release succeeds, 0 warnings, 0 errors.
• dotnet build -c Release on the full solution succeeds, all 4 projects build cleanly.
• LoginFix confirmed working end to end on a manual Wine install and on Lutris, on stock distro Wine 11.15.