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/pages/DevBranchingPage.tsx

@@ -452,7 +452,7 @@ export default function DevBranchingPage() {
         <div className="max-w-3xl mx-auto w-full flex flex-col gap-6">
           {/* Page header */}
           <div>
-            <h1 className="text-lg font-semibold text-heading">
+            <h1 className="text-lg font-heading font-semibold text-heading">
               Conversational Branching — Component Test
             </h1>
             <p className="text-sm text-secondary mt-1">