Migrate build and release flow to .NET 10

[Marc-André Moreau (mamoreau-devolutions)]

• I have read the contributing guidelines, and I agree with the Code of Conduct.
• Have you checked that there aren't other open pull requests for the same changes?
• Have you tested that the committed code can be executed without errors?
• This PR is not composed of garbage changes used to farm GitHub activity to enter potential Crypto AirDrops.

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Summary

This migrates the build, CI, and packaging flow from .NET 8 to .NET 10.

Key changes:

• add a repo-level global.json and move the central target framework to net10.0
• update GitHub Actions to use the pinned SDK and derive publish paths from Directory.Build.props
• update local build automation and winget bootstrap configs for .NET 10
• remove redundant legacy System.* package references and refresh stale VS Code launch/task config
• fix the Inno Setup privilege-mode warning by defaulting the installer to per-user mode while keeping the privilege override dialog

Validation

Local:

• dotnet format whitespace src --folder --verify-no-changes --verbosity minimal
• dotnet format style src/UniGetUI.sln --no-restore --verify-no-changes --verbosity minimal
• dotnet publish .\src\UniGetUI\UniGetUI.csproj /noLogo /p:Configuration=Release /p:Platform=arm64 -p:RuntimeIdentifier=win-arm64 --ignore-failed-sources -v minimal
• pwsh .\scripts\build.ps1 -Configuration Release -Platform x64 -SkipTests

CI:

• .NET Tests (current HEAD): https://github.com/Devolutions/UniGetUI/actions/runs/23071506744
• Build and Release dry run (current HEAD): https://github.com/Devolutions/UniGetUI/actions/runs/23071213217

Notes:

• both x64 and arm64 installer builds succeeded in CI dry run
• the previous Inno Setup PrivilegesRequired warning no longer appears in the validated rerun
• no publishing occurred during CI validation

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No linked issue.