fix: v0.3.1 – package components missing from the ShellUI.Components namespace

[Shewart]

Summary

Patch release for the ShellUI.Components NuGet package.

• Calendar, Command, DataTable, FileUpload and CarouselContent/Dots/Next/Previous had no @namespace, so they compiled into ShellUI.Components.Components and @using ShellUI.Components did not find them. They now declare ShellUI.Components; CommandPalette's workaround @using is removed.
• New test EveryPackageComponent_DeclaresThePackageNamespace.
• Removed redundant comments (same cleanup as main).
• Version 0.3.1: Directory.Build.props, fallback versions, docs, package README links, and a v0.3.1 section in the release notes.

Users who added @using ShellUI.Components.Components as a workaround must remove it; the namespace no longer exists.

Verification

• dotnet build 0 warnings, 155 tests pass.
• Packed the 0.3.1 .nupkg and built a fresh Blazor app that uses all 8 components with only @using ShellUI.Components.