Add admin help system: build-embedded markdown docs + contextual help

#37 in Riparion/riparion-cms — merged 2026-06-02

What

An in-product help system for the admin, modeled on the Dioxus docsite's markdown-sourced docs — but deliberately lighter. Docsite uses build.rs codegen (mdbook-gen → generated BookRoute enums) and a build-time Stork full-text index; that pays off for a multi-version public docs site but is overkill for a bounded, project-owned admin doc set. This keeps the good ideas (markdown authoring, routed pages, search, embeddable live components) and reuses what riparion-cms already has.

How

• Content: static, build-embedded Markdown — include_str! over src/pages/admin/help/content/*.md plus a hand-maintained HELP_ARTICLES registry. Add an article by dropping a file and appending one line.
• Rendering: articles go through the existing mdx::parse_body + render_segments pipeline, so they get .prose styling for free and can host the same live [[/name …]] embeds authors use in posts. The Embeds & smart tags article mounts a live counter to demonstrate.
• Routes: /admin/help (searchable index, client-side substring filter over the embedded registry, grouped by section) and /admin/help/:slug (one article; not-found notice with a back-link for unknown slugs). Both wrap AdminShell.
• Contextual help: a ? button in the admin header deep-links from each section to its article via help_route_for, falling back to the index for sections without one.
• Permissions: the Help sidebar entry is gated non-restrictively (the dashboard's token) since help is read-only product docs.
• Breadcrumb: two new arms (the exhaustive match enforced this at compile time); the article crumb shows the real title.

No new CSS. Reuses render_markdown/mdx, AdminShell, the .prose styles, and the surface/text catalog components.

Initial articles: getting-started, posts, pages, embeds, media, comments.

Verification

• cargo fmt --all --check clean.
• clippy -D warnings-equivalent clean on server+sqlite, server+postgres (--all-targets), and the wasm/web target. No new warnings; the pre-existing wasm dead-code lints are in server-only code, unrelated to this change.
• Manual end-to-end steps recorded locally (search filter, live embed in an article, bad-slug fallback, contextual ? deep-links, non-restrictive permission gate).

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