swarm-ui: a group pane's maximize and close are icon-only badges in its top right corner

The two controls sat among the pane's header badges as a labelled badge and a
cross glyph. They are now icon badges in the top right corner, through a
`trailing` slot of the terminal preview, so the header on the left keeps the
agent, split, subagents and state badges and a narrow pane wraps less.

frontend/packages/shared/src/icons.tsx

@@ -83,6 +83,27 @@ export function ExpandIcon() {
   );
 }
 
+// A cross, for an icon-only close trigger (a pane's "take this out of the
+// group"). Same size and stroke as `ExpandIcon`.
+export function CloseIcon() {
+  return (
+    <svg
+      width="1.3em"
+      height="1.3em"
+      viewBox="0 0 24 24"
+      fill="none"
+      stroke="currentColor"
+      stroke-width="2"
+      stroke-linecap="round"
+      stroke-linejoin="round"
+      aria-hidden="true"
+    >
+      <line x1="5" y1="5" x2="19" y2="19" />
+      <line x1="19" y1="5" x2="5" y2="19" />
+    </svg>
+  );
+}
+
 // A funnel, the standard "filter" glyph — smaller than the other two
 // (1em not 1.3em) since its first caller sits inside a table header
 // cell, not a nav trigger button; scale via the caller's own