v0.19.0 frontend: /docs/* route tree + flyout nav + sessions browser

Reorganizes the /docs surface from a single DOCS.md route into a hub
with a left-side flyout nav and three new sub-routes for the on-site
sessions browser (roadmap item #30):

  /docs                      → redirect to /docs/user-guide
  /docs/user-guide           → existing DOCS.md content
  /docs/sessions             → redirect to /docs/sessions/about
  /docs/sessions/about       → README.md of ohm-session-history
  /docs/sessions/<NNNN>      → per-session transcript index
  /docs/sessions/<NNNN>/<f>  → per-transcript view

The flyout is a persistent left sidebar on desktop and a slide-out
drawer on mobile (toggled by a ☰ button in the docs header). Its
session list is driven by the /api/docs/sessions/manifest fetch —
loading shows a skeleton; 502 shows an inline retry; empty manifest
shows only the "About" row.

Each sub-route owns its own empty-state / error handling:
  - 404 transcripts render "This transcript isn't published yet"
    with a link back to the parent session index, no JS crash.
  - 502 (gitea unreachable) renders a retry button.
  - Manifest 404 is mapped to {} server-side so the flyout renders
    cleanly with no error banner when no sessions are published yet.

Analytics (per SPEC §21):
  - new EVENTS.DOC_VIEWED ("Doc Viewed") fires on each sub-route
    mount with `section`: 'user-guide' | 'sessions/about' |
    'sessions/<NNNN>' | 'sessions/<NNNN>/<filename>'.
  - every interactive nav element carries aria-label +
    data-amp-track-name so autocapture rows are readable.

The existing v0.14.0 Docs.jsx component is dropped — its content
moved verbatim into DocsUserGuide.jsx; the new layout subsumes the
back-button + signed-out home affordances it used to carry. All
four new sub-routes reuse the existing MarkdownPreview renderer
(marked + mermaid lazy-load) so no second markdown library lands.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/DocsUserGuide.jsx

@@ -0,0 +1,45 @@
+// DocsUserGuide.jsx — v0.19.0 / roadmap item #30.
+//
+// Renders DOCS.md at `/docs/user-guide`. Was `/docs` before v0.19.0
+// (the v0.14.0 single-route Docs.jsx surface, now superseded). The
+// content path is unchanged: backend reads `DOCS.md` from disk and
+// serves it at `/api/docs`. The body is rendered via the existing
+// `MarkdownPreview` (the same component the `/philosophy` route uses,
+// so we don't introduce a second markdown library).
+
+import { useEffect, useState } from 'react'
+import MarkdownPreview from './MarkdownPreview.jsx'
+import { getDocs } from '../api.js'
+import { EVENTS, track } from '../lib/analytics'
+
+export default function DocsUserGuide() {
+  const [body, setBody] = useState('')
+  const [error, setError] = useState(null)
+  const [loading, setLoading] = useState(true)
+
+  useEffect(() => {
+    track(EVENTS.DOC_VIEWED, { section: 'user-guide' })
+  }, [])
+
+  useEffect(() => {
+    let active = true
+    getDocs()
+      .then(r => { if (active) setBody(r.body || '') })
+      .catch(e => { if (active) setError(e.message || String(e)) })
+      .finally(() => { if (active) setLoading(false) })
+    return () => { active = false }
+  }, [])
+
+  return (
+    <article className="docs-article">
+      <h1 className="docs-article-title">User guide</h1>
+      {loading && <p className="muted">Loading…</p>}
+      {error && <p className="error">Could not load the guide: {error}</p>}
+      {!loading && !error && (
+        <div className="philosophy-body">
+          <MarkdownPreview content={body} />
+        </div>
+      )}
+    </article>
+  )
+}