fix: consumer-configurable brand logo across sidebar / header / footer

[Shewart]

Summary

Second dogfood-driven fix (see docs/SHELLUI_DOGFOOD_FIXES.md). The chrome components were rendering a hardcoded rounded-square dot placeholder next to Options.SiteName, with no config path for a real brand mark — consumers had to resort to CSS background-image overrides with !important to defeat Blazor scoped-CSS attribute-selector specificity.

The fix

New ShellDocsOptions surface

public string? LogoLight { get; set; }
public string? LogoDark  { get; set; }
public string? LogoAlt   { get; set; }        // defaults to SiteName in render
public double  LogoHeight { get; set; } = 1.375;   // rem

public ShellDocsOptions SetLogo(string url, string? alt = null);
public ShellDocsOptions SetLogo(string lightUrl, string darkUrl, string? alt = null);

Consumers:

builder.Services.AddShellDocs(o =>
{
    o.SetLogo("/img/logo-light.svg", "/img/logo-dark.svg", alt: "ShellUI");
});

Shared <BrandLogo /> component

src/ShellDocs.Components/Chrome/BrandLogo.razor — renders both light + dark <img> tags as siblings when logos are configured; :root.dark .brand-logo-light { display: none } / :root.dark .brand-logo-dark { display: inline-block } handles per-theme visibility. Theme toggle is instant CSS — no re-render, no <picture> prefers-color-scheme mismatch with the class-based theme system.

Falls back to the existing <span class="brand-logo-dot"> placeholder when no logos are set — backward-compatible for the preview app and any consumer that hasn't opted in.

Chrome components consolidated

All three components (DocsSidebarHeader, DocsHeader, DocsFooter) previously carried their own .docs-*-logo-dot markup + CSS block. Now they render <BrandLogo /> and the three dot-block CSS rules are deleted. Net negative LOC.

Test plan

• dotnet build shelldocs.slnx — clean, 0 warnings, 0 errors
• dotnet test shelldocs.slnx — 125 / 125 passing (+4 in BrandLogoOptionsTests)
• Preview smoke, no SetLogo call:
  • <span class="brand-logo-dot"> renders in sidebar, header, footer — backward-compatible with existing shelldocs-preview UX
• Preview smoke, SetLogo("/img/logo-light.svg", "/img/logo-dark.svg", "ShellDocs") (temporarily wired, verified, reverted):
  • Both <img class="brand-logo brand-logo-light"> and <img class="brand-logo brand-logo-dark"> land with correct src, alt="ShellDocs", style="height: 1.375rem"
  • :root.dark .brand-logo-light { display: none } hides the light img in dark mode
  • Preview app's Program.cs reverted to its zero-config state after verification — no ambient logo added to shelldocs-preview

Files touched

• src/ShellDocs.Components/ShellDocsOptions.cs — logo props + SetLogo overloads
• src/ShellDocs.Components/Chrome/BrandLogo.razor (new) + .razor.css (new)
• src/ShellDocs.Components/Chrome/DocsSidebarHeader.razor + .razor.css
• src/ShellDocs.Components/Chrome/DocsHeader.razor + .razor.css
• src/ShellDocs.Components/Chrome/DocsFooter.razor + .razor.css
• tests/ShellDocs.Tests/BrandLogoOptionsTests.cs (new)
• docs/SHELLUI_DOGFOOD_FIXES.md — moved to Fixed