Add app icon: favicon, apple-touch icon, and top-bar logo

Generated a transparent PNG set from the source icon (a JPEG with a baked-in
checkerboard) by keying out the background on saturation — the arrow is
strongly saturated (0.47-0.68) while the grey/white background is 0.00, so
the cutout is clean. Produced static/logo.png (top-bar, natural aspect),
static/favicon.png (32px), and static/apple-touch-icon.png (180px).

Wired favicon + apple-touch links into all three page heads (feed, settings,
login) and placed the logo next to the "AgentFeed" title in the top bar.

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

templates/artifacts/_topbar.html

@@ -1,6 +1,6 @@
 <header class="topbar">
   <div class="brandline">
-    <a class="brand-link" href="{% url 'feed' %}"><h1>AgentFeed</h1></a>
+    <a class="brand-link" href="{% url 'feed' %}"><img class="brand-logo" src="/static/logo.png" alt=""><h1>AgentFeed</h1></a>
   </div>
   <div class="toolbar">
     {% if topbar_toolbar %}{% include topbar_toolbar %}{% endif %}