chore: port v0.3.0 fixes and update docs for the 0.3.0 release

[Shewart]

Summary

Brings the fixes that shipped in v0.3.0 to main and updates the docs now that 0.3.0 is the stable release.

Ported from v0.3.0

• input, alert, badge and toggle imported ….Components.UI.Variants, a namespace they don't install, so adding one of them on its own failed to build.
• chart-series now depends on chart, which brings the Blazor-ApexCharts package.
• .NET 8/9 projects named like my-app get a valid namespace (my_app). The templates write <RootNamespace>my-app</RootNamespace>, and the CLI used it as-is.
• Test: every direct target installs the namespaces and packages it imports.
• Test: namespace sanitizing.
• CI: the all-components check builds .NET 10, 9 and 8 apps with hyphenated names.

Docs

• README status: stable 0.3.0; 0.4.0 in development on main, with a link to the v0.3.0 docs. --version 0.3.0-rc.2 pins and "0.2.1 stable" wording removed.
• .NET support: the CLI works in Blazor projects on .NET 8, 9 and 10; the NuGet package targets .NET 10.
• NuGet READMEs from the 0.3.0 release, adapted for 0.4 (--dashboard, 90 components, links to main).
• Release notes: v0.3.0 and rc.3 sections, identical to the release.

Verification

• dotnet test: 220 passed.
• .NET 8 my-app: init --yes --dashboard 02 plus the fixed components builds with 0 warnings.
• Docs audit: every CLI flag and command, shellui add example and relative link checked.