Add nav-item + link-card fragments and oEmbed endpoint

- nav-item fragment handler with template
- link-card fragment handler with CID-based lookup, friendly names, batch mode
- oEmbed router at GET /oembed for media/recipe/effect/run content
- Fragment templates in app/templates/fragments/

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/templates/fragments/nav_item.html

@@ -0,0 +1,7 @@
+<div class="relative nav-group">
+  <a href="{{ artdag_url }}"
+     class="justify-center cursor-pointer flex flex-row items-center gap-2 rounded bg-stone-200 text-black p-3"
+     data-hx-disable>
+    <i class="fas fa-project-diagram text-sm"></i> art-dag
+  </a>
+</div>