feat(v5.5.0): Embeds & Media riche - universal embeds, bookmark cards, lightbox, inline previews, cover & icon

- embeds.py: provider detection/rewrite (YouTube, Vimeo, Figma, Maps, Docs,
  Loom, CodePen, Miro, Spotify, SoundCloud, Twitch, X/Twitter, Pinterest,
  Office) + resolve_embed/inline_kind/provider; POST /board/api/embed/resolve
- editor resolves pasted URLs and caches embed_src (persisted); renderer,
  public pages and MD/HTML/PDF exports prefer embed_src
- og_fetcher.py: robust meta parsing (any attribute order), favicon,
  injectable transport, network-safe fallback
- image lightbox with keyboard nav (arrows/Esc) in editor and public pages
- inline PDF/video/audio previews
- cover (URL or upload) & page icon endpoints
- fix broken editor API paths (/api/pages -> /board/api/pages) for cover,
  icon, versions, backlinks, import, move and OG metadata
- 47 tests in tests/test_v55.py; full suite 444 green; ruff clean
- version 5.11.2

static/css/app.css

@@ -4747,3 +4747,19 @@ button.breadcrumb-current { max-width: 260px; }
 .aici-result a { color: var(--accent,#2383E2); }
 .aici-fb:disabled { opacity:.4; cursor:default; }
 @media (max-width:768px){ #_aiC { width: 92vw; } }
+
+/* v5.5.0 — Image lightbox */
+.fd-lightbox { user-select: none; }
+.fd-lightbox-img { animation: fdLbIn .12s ease-out; }
+@keyframes fdLbIn { from { opacity: 0; transform: scale(.98); } to { opacity: 1; transform: scale(1); } }
+.fd-lightbox-nav {
+  position: absolute; top: 50%; transform: translateY(-50%);
+  width: 48px; height: 48px; border-radius: 50%;
+  border: none; background: rgba(255,255,255,.12); color: #fff;
+  font-size: 30px; line-height: 1; cursor: pointer;
+  display: flex; align-items: center; justify-content: center;
+  transition: background .15s ease;
+}
+.fd-lightbox-nav:hover { background: rgba(255,255,255,.25); }
+.fd-lightbox-nav[data-nav="prev"] { left: 24px; }
+.fd-lightbox-nav[data-nav="next"] { right: 24px; }