swarm-ui: give Panel an optional header icon slot

Adds an `icon` prop to Panel (small emoji glyph left of the title,
aria-hidden, chosen per panel with no default) and wires it into every
current Panel caller: hives (bee), create-agent's form (robot) and info
panel (identity card, moved off the info panel's body copy where it
started as a one-off), jobs (puzzle piece), the components gallery
itself (toolbox), and the 404 page (compass). Adds a components-page
demo section and a whimsy-section pointer in the design guide.

Closes: #3508

frontend/packages/swarm-ui/src/App.tsx

@@ -10,7 +10,7 @@ import { Panel } from './ui/panel/Panel.js';
 
 function NotFound() {
   return (
-    <Panel title="404">
+    <Panel title="404" icon="🧭">
       <p>no route here.</p>
     </Panel>
   );