Integrate federation app with shared menu/header system

Replace standalone base.html with the shared _types/root layout.
Social pages get a second nav row via _types/social/index.html.
Root / becomes a blank page with shared chrome. Auth pages use
the shared layout without the social nav bar.

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

templates/federation/choose_username.html

@@ -1,6 +1,6 @@
-{% extends "federation/base.html" %}
+{% extends "_types/social/index.html" %}
 {% block title %}Choose Username — Rose Ash{% endblock %}
-{% block content %}
+{% block social_content %}
 <div class="py-8 max-w-md mx-auto">
   <h1 class="text-2xl font-bold mb-2">Choose your username</h1>
   <p class="text-stone-600 mb-6">