Allow custom PinGet executable paths

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• Honor existing custom manager executable paths even when they are not part of the detected PATH candidates.
• Add a Browse... action to both WinUI and Avalonia package-manager settings so users can select pinget.exe outside PATH.
• Keep secure custom-path gating intact and preserve fallback behavior for missing saved paths.
• Update package-manager tests to cover saved custom paths with no detected candidates and saved paths outside the candidate list.

Validation

• dotnet test src\UniGetUI.PackageEngine.Tests\UniGetUI.PackageEngine.Tests.csproj --no-restore --verbosity minimal --nologo -tl:off
• dotnet build src\UniGetUI.Avalonia\UniGetUI.Avalonia.csproj --no-restore --verbosity minimal --nologo -tl:off /p:UseSharedCompilation=false
• dotnet restore src\UniGetUI\UniGetUI.csproj --verbosity minimal --nologo -tl:off
• dotnet build src\UniGetUI\UniGetUI.csproj --no-restore --verbosity minimal --nologo -tl:off /p:Platform=x64 /p:UseSharedCompilation=false
• Launched the x64 debug app and confirmed it delegated to the running modern UniGetUI.Avalonia process.