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/ComponentsPage.tsx

@@ -88,7 +88,7 @@ function RefreshIntervalPickerSample() {
 
 export function ComponentsPage() {
   return (
-    <Panel title="components">
+    <Panel title="components" icon="🧰">
       <p class="components-intro">
         Every primitive in <code>src/ui/</code>, shown in each mode it supports. Sample data
         only — nothing here calls the API.
@@ -101,6 +101,11 @@ export function ComponentsPage() {
         <Sample label="without title">
           <Panel>panel body content, no title</Panel>
         </Sample>
+        <Sample label="with icon (per panel, no default — see the doc comment)">
+          <Panel title="example title" icon="✨">
+            panel body content
+          </Panel>
+        </Sample>
         <Sample label="with actions (e.g. HivesPage's refresh picker)">
           <Panel
             title="example title"