docs: audit corrections, coverage pages, screenshots, and em-dash removal

Finalizes the docs section after an adversarial audit (code review + per-page
fact-check + coherence critic) and two content requests.

- Accuracy: fixed 10 invented cross-link slugs, the admin-vs-board gating claims
  the verifiers caught (permissions matrix, /settings, invites, onboarding,
  connect-payments, proposal withdraw, Stripe reconciliation, member fields,
  door.operate), and 2 issues in the new pages (project area-at-creation, forum
  thread-delete role).
- Coverage: added 5 how-to pages so every module is documented (recurring Stripe
  dues, tasks & chores, projects, comms & forum, ops knowledge base + secrets
  vault). 34 pages total.
- Screenshots: 12 captioned app screenshots under public/docs-media/, embedded
  as figures. Each tutorial now has 2-3 step screenshots.
- Style: removed all em-dashes/en-dashes/ellipses from the docs (natural ASCII
  punctuation).
- Handoff (docs/HANDOFF-audit-cleanup.md) updated with the current state.

typecheck 0 · lint 0 errors · build ok (all 34 /docs pages prerendered)

app/(docs)/docs.css

@@ -259,6 +259,22 @@
 }
 .docs-prose blockquote p { margin: 10px 0; }
 .docs-prose hr { border: none; border-top: 1px solid var(--doc-border); margin: 28px 0; }
+.docs-figure { display: block; margin: 22px 0; }
+.docs-figure img {
+  display: block;
+  max-width: 100%;
+  height: auto;
+  border: 1px solid var(--doc-border);
+  border-radius: 10px;
+  background: var(--doc-surface);
+}
+.docs-figcaption {
+  display: block;
+  margin-top: 8px;
+  font-size: 12px;
+  color: var(--doc-text-muted);
+  text-align: center;
+}
 .docs-prose table {
   width: 100%;
   border-collapse: collapse;