swarm-ui: show each agent's icon on its card

Each agent card now leads with the agent's icon, loaded as an `<img>`
from `GET /api/agents/<name>/icon`: the same 5em square, background and
fallback as the hive dashboard's container row. An agent with no icon
(the route's 404), or any other failed load, shows the dimmed hyperhive
mark (`/favicon.svg`) instead of a broken image.

Only ever an `<img>`, never inline markup: the body is an agent-authored
SVG, and an image load does not run its script.

frontend/packages/swarm-ui/src/pages/agents/AgentCard.css

@@ -22,3 +22,37 @@
   white-space: normal;
   word-break: break-word;
 }
+/* Icon beside the name/message, sized and dimmed like the hive
+   dashboard's `.container-icon` (dashboard.css). */
+.ui-agent-card-row {
+  display: flex;
+  align-items: flex-start;
+  gap: 0.7em;
+}
+.ui-agent-card-body {
+  display: flex;
+  flex-direction: column;
+  gap: 0.35em;
+  flex: 1;
+  min-width: 0;
+}
+.ui-agent-card-icon {
+  position: relative;
+  overflow: hidden;
+  flex: none;
+  width: 5em;
+  aspect-ratio: 1;
+  border-radius: 6px;
+  background-color: color-mix(in srgb, var(--crust) 60%, transparent);
+}
+.ui-agent-card-icon > img {
+  position: absolute;
+  inset: 0;
+  width: 100%;
+  height: 100%;
+  object-fit: contain;
+}
+.ui-agent-card-icon-none {
+  filter: grayscale(1);
+  opacity: 0.4;
+}