feat(seo): add per-page descriptions and canonical metadata

[BenjaminMichaelis]

Why

Every page currently receives the same site-wide description, and og:url always points at the homepage. This makes chapter and static-page metadata misleading to search engines and link previews. It also has to be correct on both essentialcsharp.com and dev.essentialcsharp.com.

What changed

• Give public MVC actions page-specific descriptions. Chapter pages read ParserCli-generated chapter descriptions when present, with a heading-based fallback when the generated file is absent.
• Emit exactly one canonical <link> and a matching og:url on every successful, non-Identity page, excluding query strings. The origin always comes from SiteSettings:BaseUrl.
  • Chapter pages keep the path from the parser-generated canonical (so aliases such as /hello-world point to /introducing-c), re-based from the parser's hardcoded production origin onto the configured origin.
  • The parser's own canonical tag is stripped from the injected head.
• Share the selected description across HTML, Open Graph, and Twitter metadata, while retaining the layout fallback for pages without an override.
• Cover canonical and description output for both production and dev origins with functional tests; document the metadata contract.

Supersedes #1130

This reconciles #1130 and addresses its review findings:

• Duplicate/conflicting canonical on aliases: fixed. There is one canonical, and it points to the primary page.
• X-Forwarded-Host without an allowlist: avoided. The request Host is not used for canonicals, so XForwardedHost is not enabled. That flag would also have let a trusted proxy poison emailed reset/confirmation links.
• XForwardedHost is a no-op under ASPNETCORE_FORWARDEDHEADERS_ENABLED=true: moot for the same reason.

Also addresses this PR's own review comment: metadata tests now pin SiteSettings:BaseUrl instead of depending on appsettings.Development.json.

Deployment note

Each environment must set SiteSettings__BaseUrl to its own origin, for example https://dev.essentialcsharp.com for dev. Production already depends on this for the www→apex redirect.

Validation

The full Web test suite passes locally (186 tests), built with /p:SkipFrontendBuild=true. The local frontend build is blocked by an unrelated stale @zxcvbn-ts/core install.

[Copilot]

🟡 Changes recommended

Static-page metadata tests expect production URLs but the default test configuration supplies a localhost origin.

1 open issue

• Set production BaseUrl explicitly in test host configuration · New

What changed in this PR

Adds page-specific SEO metadata while preserving parser-generated chapter canonicals.

Changes:

• Adds public-page descriptions and optional generated chapter descriptions with heading fallbacks.
• Shares descriptions across metadata formats and aligns Open Graph URLs with canonicals.
• Adds metadata tests and documents the contract.

File	Description
README.md	Documents metadata selection and fallbacks.
EssentialCSharp.Web/​Views/​Shared/​_Layout.cshtml	Renders descriptions and canonical metadata.
EssentialCSharp.Web/​Services/​ChapterDescriptionService.cs	Loads generated descriptions with heading fallbacks.
EssentialCSharp.Web/​Program.cs	Registers the description service.
EssentialCSharp.Web/​Controllers/​McpSetupController.cs	Adds the setup-page description.
EssentialCSharp.Web/​Controllers/​HomeController.cs	Sets descriptions and preserves chapter canonicals.
EssentialCSharp.Web.Tests/​FunctionalTests.cs	Checks rendered descriptions and canonicals.
EssentialCSharp.Web.Tests/​ChapterDescriptionServiceTests.cs	Tests generated descriptions and fallbacks.

_{Review effort: Balanced}

[Copilot]

🟢 Approval recommended

Static review identified no unresolved blocking issues; tests were not executed during this review.

No open issues 🎉

1 resolved since last review

• Set production BaseUrl explicitly in test host configuration

_{Review effort: Balanced}