swarm-ui: drop the redundant outer dialog card, move close into the panel header

Dialog and Panel both drew their own bordered/backgrounded card in the
same --bg-elev, so a Panel-based dialog (create-agent, link-matrix-account)
rendered as two concentric cards with a floating close button on the
outer one and no purpose to it.

Give Dialog a "plain" mode that drops its own card chrome (border,
background, padding) and floating close button, and give Panel an
optional onClose that renders a close button at the end of its own
header row instead. AgentsPage's two Panel-backed dialogs now use
plain + Panel's onClose, so the Panel is the dialog's only visible
card. ConfirmDialog (no Panel of its own) is unaffected — plain
defaults to false, unchanged card + floating close button.

Added a ComponentsPage sample demonstrating the plain + onClose
pairing. Verified both dialog modes via a real headless-chromium
screenshot (plain dialog: single card, close button in the header
bar; default dialog: unchanged floating close button).

frontend/packages/swarm-ui/src/pages/LinkMatrixAccountForm.tsx

@@ -63,9 +63,14 @@ const MODE_OPTIONS = [
 export function LinkMatrixAccountForm({
   hive,
   agent,
+  onClose,
 }: {
   hive: string;
   agent: string;
+  /** Rendered as the panel header's own close button — this form always
+   * mounts inside a `plain` `Dialog` (see `AgentsPage`), which no longer
+   * floats its own. */
+  onClose?: () => void;
 }) {
   const [account, setAccount] = useState("");
   const [mode, setMode] = useState<Mode>("token");
@@ -127,7 +132,11 @@ export function LinkMatrixAccountForm({
   }
 
   return (
-    <Panel title={`link a matrix account — ${agent}`} icon="🔗">
+    <Panel
+      title={`link a matrix account — ${agent}`}
+      icon="🔗"
+      onClose={onClose}
+    >
       <p>
         Writes the credential to the swarm secret store and notifies{" "}
         <strong>{hive}</strong> to deliver it. The agent's own matrix daemon