agent icon: render on dashboard + per-agent web UI

Consumes the GET /icon endpoint from #139:

- Dashboard: each container card shows the agent's icon next to its
  name (26px). Loaded from <agent-url>/icon; onerror hides it for a
  stopped container whose web server isn't answering.
- Per-agent web UI: the agent's icon next to the page title (40px),
  and /icon as the favicon on the index, stats, and screen pages.

/icon always returns an image (configured SVG or the default
hyperhive logo), so no presence check is needed.

Closes #140

hive-ag3nt/assets/screen.html

@@ -4,6 +4,7 @@
 <meta charset="UTF-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>screen</title>
+<link rel="icon" type="image/svg+xml" href="/icon">
 <style>
 /* Catppuccin Mocha palette (mirrors base.css) */
 :root {