feat(ui): the key logo replaces the crown; favicon

The operator's concept, redrawn as a single-colour SVG in the accent gold:
a key whose bow is a cloverleaf holding three connected nodes, hanging on a
loop threaded through a keyring and swinging into the ring's middle, with a
small sparkle.

- static/logo/steward-logo.svg: the full mark (node graph in the bow).
- static/logo/steward-mark.svg: the small mark for 16-32 px (heavier lines,
  one centre node, two teeth); the node graph turns to mush below ~40 px.
  Used in the sidebar brand (was an inline crown) and as the favicon on every
  page and on the public share view.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/templates/base.html

@@ -19,6 +19,7 @@
       }, true);
     </script>
     <title>{% block title %}Steward{% endblock %}</title>
+    <link rel="icon" type="image/svg+xml" href="{{ url_for('static', filename='logo/steward-mark.svg') }}">
     {# Every asset is served by Steward itself: it runs with no outbound network (rule 164). #}
     <script src="{{ url_for('static', filename='vendor/htmx-2.0.4.min.js') }}"></script>
     <script src="{{ url_for('static', filename='vendor/chart-4.4.4.umd.min.js') }}"></script>
@@ -37,14 +38,7 @@
 {% if session.user_id is defined %}
   <aside class="sidebar" id="sidebar">
     <a href="/" class="brand">
-      <svg width="22" height="22" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg" aria-hidden="true">
-        <circle cx="12" cy="12" r="10.5" fill="none" stroke="#c8a840" stroke-width="1.5"/>
-        <circle cx="12" cy="12" r="9.3" fill="#8a8a92" opacity="0.1"/>
-        <path d="M7 10 L7 5.8 L9 7.8 L10.5 5 L12 7.2 L13.5 5 L15 7.8 L17 5.8 L17 10 Z" fill="#c8a840"/>
-        <ellipse cx="12" cy="15.5" rx="5.2" ry="1" fill="#c8a840"/>
-        <rect x="9.6" y="15.5" width="1" height="3.8" fill="#c8a840"/>
-        <rect x="13.4" y="15.5" width="1" height="3.8" fill="#c8a840"/>
-      </svg>
+      <img src="{{ url_for('static', filename='logo/steward-mark.svg') }}" width="22" height="22" alt="">
       Steward
     </a>
     <nav class="side-nav" aria-label="Primary">