feat(HAI-004): add settings trigger to Header and wire into App

packages/dashboard/app/components/Header.tsx

@@ -1,10 +1,19 @@
-export function Header() {
+interface HeaderProps {
+  onOpenSettings?: () => void;
+}
+
+export function Header({ onOpenSettings }: HeaderProps) {
   return (
     <header className="header">
       <div className="header-left">
         <h1 className="logo">hai</h1>
         <span className="logo-sub">board</span>
       </div>
+      <div className="header-actions">
+        <button className="btn-icon" onClick={onOpenSettings} title="Settings">
+          ⚙
+        </button>
+      </div>
     </header>
   );
 }