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/api.js

@@ -720,6 +720,60 @@ export async function getDocs() {
   return jsonOrThrow(await fetch('/api/docs'))
 }
 
+// ---------------------------------------------------------------------------
+// v0.19.0 / roadmap item #30 — /api/docs/sessions/* surface
+// ---------------------------------------------------------------------------
+//
+// The framework mediates reads against the public
+// `wiggleverse/ohm-session-history` gitea repo so the rendered
+// `/docs/sessions/*` surface inherits the same chrome as
+// `/docs/user-guide`. Three text-bearing endpoints return markdown
+// (Content-Type: text/markdown) and the manifest returns JSON. We
+// wrap each into a small helper.
+//
+// 404 from `getSessionAbout` / `getSessionTranscript` / `getSessionIndex`
+// throws an Error with `.status === 404` so the UI can render its own
+// empty-state. 502 (gitea unreachable) throws `.status === 502` so
+// the UI can offer a retry button.
+
+export async function getSessionsManifest() {
+  // Manifest 404 is mapped server-side to HTTP 200 + `{}` so this
+  // helper never throws on the empty-state path.
+  return jsonOrThrow(await fetch('/api/docs/sessions/manifest'))
+}
+
+async function _textOrThrow(res) {
+  if (!res.ok) {
+    let detail = ''
+    try {
+      const body = await res.json()
+      detail = body.detail || JSON.stringify(body)
+    } catch {
+      detail = await res.text()
+    }
+    const error = new Error(detail || `HTTP ${res.status}`)
+    error.status = res.status
+    throw error
+  }
+  return res.text()
+}
+
+export async function getSessionsAbout() {
+  return _textOrThrow(await fetch('/api/docs/sessions/about'))
+}
+
+export async function getSessionTranscript(nnnn, filename) {
+  return _textOrThrow(await fetch(
+    `/api/docs/sessions/${encodeURIComponent(nnnn)}/${encodeURIComponent(filename)}`
+  ))
+}
+
+export async function getSessionIndex(nnnn) {
+  return jsonOrThrow(await fetch(
+    `/api/docs/sessions/${encodeURIComponent(nnnn)}/index`
+  ))
+}
+
 // ---------------------------------------------------------------------------
 // Slice 7: admin neighborhood (§17 admin/* + user search for the §15.8 mute
 // typeahead).