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-c0re/assets/dashboard.css

@@ -95,6 +95,12 @@ a:hover {
   font-size: 1.05em;
   font-weight: bold;
 }
+.container-row .head .container-icon {
+  width: 26px;
+  height: 26px;
+  border-radius: 4px;
+  flex-shrink: 0;
+}
 .container-row .head .meta { margin-left: auto; }
 .container-row .actions {
   display: flex;