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>
+  )
+}