feat: consistent AP logo, Mastodon syndication detection

- Replace network-diagram SVG with official ActivityPub logo in post footer
- Replace person-plus SVG with AP logo in fediverse follow widget
- Replace Mastodon icon with AP logo in share widget, use #a730b8 purple
- AP icon brand color: #f1027e → #a730b8 in social-icon macro
- Detect Mastodon syndication URLs (/@username pattern) for proper logo pill

Confab-Link: http://localhost:8080/sessions/c5b1471e-b046-44d9-b94f-ab5e68fae7cc

_includes/components/widgets/fediverse-follow.njk

@@ -25,9 +25,7 @@
      @click="handleClick($event)"
      class="inline-flex items-center gap-2 px-4 py-2 rounded-lg bg-[#a730b8]/10 text-[#a730b8] hover:bg-[#a730b8]/20 transition-colors text-sm font-medium cursor-pointer"
      title="Follow from your fediverse instance (Shift+click to change)">
-    <svg class="w-4 h-4" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true">
-      <path d="M16 21v-2a4 4 0 0 0-4-4H5a4 4 0 0 0-4 4v2"/><circle cx="8.5" cy="7" r="4"/><line x1="20" y1="8" x2="20" y2="14"/><line x1="23" y1="11" x2="17" y2="11"/>
-    </svg>
+    <svg class="w-4 h-4" viewBox="0 0 24 24" fill="currentColor" aria-hidden="true"><path d="M13.09 4.43L24 10.73v2.51L13.09 19.58v-2.51L21.83 12 13.09 6.98v-2.55zM13.09 9.49L17.44 12l-4.35 2.51V9.49z"/><path d="M10.91 4.43L0 10.73v2.51l8.74-5.03v10.09l2.18 1.28V4.43zM6.56 12L2.18 14.51l4.35 2.51V12z"/></svg>
     <span>Follow on the Fediverse</span>
   </a>
   {% set modalTitle = "Follow on the Fediverse" %}