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/pages/HivesPage.tsx

@@ -115,6 +115,7 @@ export function HivesPage() {
   return (
     <Panel
       title="hives"
+      icon="🐝"
       actions={<RefreshIntervalPicker id="hives-refresh" value={intervalMs} onChange={setIntervalMs} />}
     >
       {error ? <ApiErrorPanel context="failed to load the hive roster" problem={error} /> : null}