v0.2.1: top-bar avatar, You/Admin settings groups, admin models

- Replace the gear/username button with a round initials avatar (per-user
  deterministic color via a new daybook_extras template filter); dropdown
  now shows the username and keeps Settings/About/theme/logout. Sets up an
  optional profile photo later.
- Regroup the settings nav into "You" (Account, API Keys, Storage, Demo
  Data) and a superuser-only "Admin" (Users), About separate — the split
  now reflects who each setting affects, not a flat list.
- Register ApiKey (key read-only) and TagColor in Django admin so /admin/
  is complete as the operator break-glass tool, distinct from the themed
  Settings product surface.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/artifacts/_topbar.html

@@ -1,3 +1,4 @@
+{% load daybook_extras %}
 <header class="topbar">
   <div class="brandline">
     <a class="brand-link" href="{% url 'feed' %}"><img class="brand-logo" src="/static/logo.png" alt=""><h1>AgentFeed</h1></a>
@@ -6,12 +7,12 @@
     {% if topbar_toolbar %}{% include topbar_toolbar %}{% endif %}
   </div>
   <div class="topbar-actions">
-    <button class="icon-button" type="button" data-user-menu-toggle aria-haspopup="true" aria-expanded="false" title="{{ request.user.username }}">
-      &#9881; {{ request.user.username }}
+    <button class="avatar-button" type="button" data-user-menu-toggle aria-haspopup="true" aria-expanded="false" title="{{ request.user.username }}" aria-label="{{ request.user.username }} menu">
+      <span class="avatar" style="background: {{ request.user.username|avatar_color }};">{{ request.user.username|avatar_initial }}</span>
     </button>
     <div class="user-menu-dropdown" hidden data-user-menu>
+      <div class="user-menu-name">{{ request.user.username }}</div>
       <a href="{% url 'settings-account' %}">Settings</a>
-      <a href="{% url 'settings-demo-data' %}">Demo Data</a>
       <a href="{% url 'settings-about' %}">About</a>
       <button type="button" data-theme-toggle>Dark mode</button>
       <form method="post" action="{% url 'logout' %}">{% csrf_token %}<button type="submit">Log out</button></form>