fix: add font-heading to branching component headings

Match production design system — Bricolage Grotesque on h1/h2 elements.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/session/HandoffModal.tsx

@@ -53,7 +53,7 @@ export function HandoffModal({ onClose, onSubmit }: HandoffModalProps) {
       >
         {/* Header */}
         <div className="flex items-center justify-between px-4 py-3 border-b border-default">
-          <h2 className="text-sm font-semibold text-heading">Hand Off Session</h2>
+          <h2 className="text-sm font-heading font-semibold text-heading">Hand Off Session</h2>
           <button
             type="button"
             onClick={onClose}