Add option to hide package list illustrations

[hak0]

• 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?
• Have you confirmed that this issue is caused by UniGetUI itself, and not by the package manager or the package involved?

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Summary

• add a Show illustrations on package lists toggle under Settings → User interface preferences → Package lists
• persist the preference through the new DisablePackageIllustrations setting
• hide the shared loading illustration and page-specific empty-state illustrations when the option is disabled
• avoid loading empty-state bitmap assets when illustrations are disabled
• keep illustrations enabled by default to preserve the existing experience

Motivation

The colorful loading and empty-state illustrations can be visually prominent and may distract from package-list content. Users who prefer a simpler, more professional interface currently have no way to remove them.

This is specific to UniGetUI's presentation layer and does not involve an underlying package manager.

Behavior

When Show illustrations on package lists is disabled and UniGetUI is restarted:

• the loading illustration is not shown
• empty-state illustrations on Software Updates, Installed Packages, and Package Bundles are not shown
• empty-state and loading text remains visible
• package icons and other functional imagery are unaffected

A restart warning is shown because package page view models read this preference when they are created.

Screenshot

The new setting is available under Settings → User interface preferences → Package lists:

Validation

Passed:

• pwsh ./scripts/install-git-hooks.ps1
• dotnet format whitespace src --folder --verify-no-changes
• dotnet restore src/UniGetUI.Windows.slnx
• dotnet format style src/UniGetUI.Windows.slnx --no-restore --verify-no-changes
• dotnet build src/UniGetUI.Avalonia/UniGetUI.Avalonia.csproj --no-restore --configuration Release --nologo /p:Platform=x64
• published and launched the win-x64 Avalonia build
• manually confirmed the new setting, enabled/disabled presentation, and restart warning
• confirmed no duplicate open PR for this change
• git diff --check

Full-suite note:

• dotnet test src/UniGetUI.Windows.slnx --no-restore --verbosity q --nologo /p:Platform=x64 completed with three failures in the existing network-dependent UniGetUI.Core.IconEngine.Tests.IconCacheEngineTests tests:
  • TestCacheEngineForIconUri
  • TestCacheEngineForSha256
  • TestCacheEngineForPackageSize
• the same three failures reproduced when HTTP_PROXY/HTTPS_PROXY were set; UniGetUI's HTTP handler reads its own file-based proxy settings instead of those environment variables
• the referenced test images are reachable through the proxy and their sizes and SHA256 hashes match the test constants
• these failures are unrelated to the package-list illustration setting

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Closes #5012