fix: 404 for unknown pages, 404.html in static builds, static sidebar collapse (0.1.14-alpha)

[Shewart]

Summary

Unknown docs pages now answer HTTP 404, static exports get a real 404.html, and the desktop sidebar collapse works without a Blazor runtime, so search is the only chrome that still needs one. Bumps to 0.1.14-alpha.

Added

• <DocsNotFound />, the docs page's not-found view, used by the init template and the example site. It answers 404.
  • The status is set in Response.OnStarting: .NET 10 sends an empty body for a page whose status is already 404, unless the app has a NotFoundPage or status-code re-execution.
  • Path, HomeHref and HomeLabel override the message and the link back. The link defaults to where /docs leads.
• shelldocs build writes 404.html from the site's own not-found view, for GitHub Pages, Netlify and Cloudflare Pages. --spa-fallback still replaces it with index.html.

Changed

• Desktop sidebar collapse:
  • The toggle buttons are [data-sidebar-collapse-toggle], handled by shelldocs.js, which flips [data-sidebar-collapsed] on the page shell.
  • The choice is remembered in localStorage and restored before the sidebar can flash open.
  • The docs-sidebar-collapsed class is gone. SidebarCollapseState is still registered but unused.
• shelldocs build fails on a page that renders as not found, whether it answered 404 or rendered <DocsNotFound>.

Upgrading

• Replace the not-found branch in DocsPage.razor with <DocsNotFound />.
• Restyle .docs-sidebar-collapsed as [data-sidebar-collapsed="true"].

Testing

• Release build 0 warnings; 343 tests pass, 6 new: the 404 timing, the not-found view, the template, build detection, collapse markup and its JS. Each commit builds and passes on its own.
• Example site, interactive and as a shelldocs build static export:
  • unknown URLs answer 404 with the full layout;
  • collapse and expand are remembered across pages without a server;
  • 404.html is written and renders in the site layout.
• Pack dry-run: all six packages at 0.1.14-alpha.
• ShellUI docs site built against main and against this branch: home identical; the other 241 pages differ only by the collapse markup.