feat(docs): add a Diátaxis documentation section to the public site

A new public /docs area covering the whole platform, organized by Diátaxis:
tutorials (learning), how-to guides (tasks), reference (facts), and explanation
(the why). 29 pages, each grounded in the actual routes, actions, schema, and
UI — verified against the code and corrected for accuracy (e.g. admin-only vs
admin+board gating, where the permissions matrix actually lives, how Stripe
reconciliation matches).

Implementation:
- lib/docs/nav.ts is the single source of truth (categories + pages), driving
  the sidebar, static params, and prev/next.
- Content authored as content/docs/<category>/<slug>.md, read at build time
  (the routes are statically generated), rendered by <DocsMarkdown> — reuses the
  existing react-markdown/GFM stack, no new dependencies.
- (docs) route group with its own theme (docs.css) matching the public site's
  dark / monospace / lime aesthetic; sidebar with active-state, breadcrumb,
  prev/next, and a Diátaxis home. Public — signed-in users can read it too.
- "Docs" link added to the landing nav.

typecheck 0 · lint 0 errors · build ok (all /docs pages prerendered)

app/(docs)/docs/[category]/[slug]/page.tsx

@@ -0,0 +1,86 @@
+import Link from 'next/link'
+import { notFound } from 'next/navigation'
+import { DOC_CATEGORIES, findPage, flatPages } from '@/lib/docs/nav'
+import { loadDocContent } from '@/lib/docs/load'
+import { DocsMarkdown } from '@/components/docs/docs-markdown'
+import { DocsSidebar } from '@/components/docs/docs-sidebar'
+
+export function generateStaticParams() {
+  return DOC_CATEGORIES.flatMap(cat =>
+    cat.pages.map(page => ({ category: cat.id, slug: page.slug })),
+  )
+}
+
+export const dynamicParams = false
+
+type Params = { category: string; slug: string }
+
+export async function generateMetadata({ params }: { params: Promise<Params> }) {
+  const { category, slug } = await params
+  const found = findPage(category, slug)
+  if (!found) return {}
+  return { title: found.page.title, description: found.page.summary }
+}
+
+export default async function DocPage({ params }: { params: Promise<Params> }) {
+  const { category, slug } = await params
+  const found = findPage(category, slug)
+  if (!found) notFound()
+  const { category: cat, page } = found
+
+  const content = await loadDocContent(cat.id, page.slug)
+
+  const flat = flatPages()
+  const idx = flat.findIndex(p => p.categoryId === cat.id && p.slug === page.slug)
+  const prev = idx > 0 ? flat[idx - 1] : null
+  const next = idx >= 0 && idx < flat.length - 1 ? flat[idx + 1] : null
+
+  return (
+    <main className="docs-shell">
+      <DocsSidebar />
+      <article className="docs-content">
+        <nav className="docs-breadcrumb" aria-label="Breadcrumb">
+          <Link href="/docs">Docs</Link>
+          <span>/</span>
+          <span>{cat.label}</span>
+        </nav>
+
+        <header className="docs-page-head">
+          <p className="docs-eyebrow">{cat.label}</p>
+          <h1 className="docs-page-title">{page.title}</h1>
+          <p className="docs-page-summary">{page.summary}</p>
+        </header>
+        <hr className="docs-rule" />
+
+        {content ? (
+          <DocsMarkdown content={content} />
+        ) : (
+          <div className="docs-prose">
+            <p style={{ color: 'var(--doc-text-muted)' }}>
+              This page is being written. Check back shortly.
+            </p>
+          </div>
+        )}
+
+        <nav className="docs-pager" aria-label="Pagination">
+          {prev ? (
+            <Link href={`/docs/${prev.categoryId}/${prev.slug}`} className="docs-pager-prev">
+              <span className="docs-pager-dir">← Previous</span>
+              <span className="docs-pager-title">{prev.title}</span>
+            </Link>
+          ) : (
+            <span />
+          )}
+          {next ? (
+            <Link href={`/docs/${next.categoryId}/${next.slug}`} className="docs-pager-next">
+              <span className="docs-pager-dir">Next →</span>
+              <span className="docs-pager-title">{next.title}</span>
+            </Link>
+          ) : (
+            <span />
+          )}
+        </nav>
+      </article>
+    </main>
+  )
+}

app/(docs)/docs/page.tsx

@@ -0,0 +1,53 @@
+import Link from 'next/link'
+import { DOC_CATEGORIES } from '@/lib/docs/nav'
+
+export const metadata = {
+  title: 'Docs',
+  description:
+    'Everything you need to run your space with hackerspace.sh — organized as tutorials, how-to guides, reference, and explanation.',
+}
+
+export default function DocsHomePage() {
+  return (
+    <main className="docs-shell" style={{ gridTemplateColumns: '1fr' }}>
+      <div className="docs-content" style={{ padding: 0 }}>
+        <section className="docs-home-hero">
+          <p className="docs-eyebrow">Documentation</p>
+          <h1 className="docs-home-title">
+            Run your space, <em>documented</em>
+          </h1>
+          <p className="docs-home-lede">
+            Everything hackerspace.sh does, written down. Organized the{' '}
+            <a
+              href="https://diataxis.fr"
+              target="_blank"
+              rel="noopener noreferrer"
+              style={{ color: 'var(--doc-accent)', textDecoration: 'underline' }}
+            >
+              Diátaxis
+            </a>{' '}
+            way: learn with tutorials, get things done with how-to guides, look up the
+            details in reference, and understand the why in explanation.
+          </p>
+        </section>
+
+        <div className="docs-cat-grid">
+          {DOC_CATEGORIES.map(cat => (
+            <section key={cat.id} className="docs-cat-card">
+              <p className="docs-cat-eyebrow">{cat.tagline}</p>
+              <h2 className="docs-cat-title">{cat.label}</h2>
+              <p className="docs-cat-diataxis">{cat.diataxis}</p>
+              <div className="docs-cat-list">
+                {cat.pages.map(page => (
+                  <Link key={page.slug} href={`/docs/${cat.id}/${page.slug}`}>
+                    {page.title}
+                  </Link>
+                ))}
+              </div>
+            </section>
+          ))}
+        </div>
+      </div>
+    </main>
+  )
+}