Upgrade from .NET 5.0 to .NET 8.0 LTS

[Copilot]

.NET 5.0 is out of support. This upgrades all projects to .NET 8.0 LTS.

Changes

Project files

• Update TargetFramework from net5.0 to net8.0 across all projects
• Update AnalysisLevel from 5.0 to 8.0
• Blazor packages: 5.0.* → 8.0.*
• Test packages: xunit to 2.9.*, Microsoft.NET.Test.Sdk to 17.11.*
• CodeAnalysis packages: 4.11.* / 3.11.* for .NET 8 compatibility
• Source generator remains netstandard2.0 (required for Roslyn compatibility)

Test fix

• MemoryGameModelTest.cs: Change AnimalCard.Create() → CardHelpers.Create() (pre-existing bug that now surfaces with stricter compilation)

All 6 tests pass. No security vulnerabilities detected.

Original prompt

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This section details on the original issue you should resolve

<issue_title>upgrade</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

Custom agent used: upgrade
upgrades a project's framework or version

• Fixes upgrade #10

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