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/link_card.html

@@ -0,0 +1,22 @@
+<a href="{{ link }}" class="block rounded border border-stone-200 bg-white hover:bg-stone-50 transition-colors no-underline" data-fragment="link-card" data-app="artdag" data-hx-disable>
+  <div class="flex flex-row items-center gap-3 p-3">
+    <div class="flex-shrink-0 w-10 h-10 rounded bg-stone-100 flex items-center justify-center text-stone-500">
+      {% if content_type == "recipe" %}
+        <i class="fas fa-scroll text-sm"></i>
+      {% elif content_type == "effect" %}
+        <i class="fas fa-magic text-sm"></i>
+      {% elif content_type == "run" %}
+        <i class="fas fa-play-circle text-sm"></i>
+      {% else %}
+        <i class="fas fa-cube text-sm"></i>
+      {% endif %}
+    </div>
+    <div class="flex-1 min-w-0">
+      <div class="font-medium text-stone-900 text-sm truncate">{{ title }}</div>
+      {% if description %}
+        <div class="text-xs text-stone-500 clamp-2">{{ description }}</div>
+      {% endif %}
+      <div class="text-xs text-stone-400 mt-0.5">{{ content_type }} &middot; {{ cid[:12] }}&hellip;</div>
+    </div>
+  </div>
+</a>

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>