feat: add authentication token handling for design persistence

app/pages/designer.vue

@@ -128,8 +128,12 @@ const persistDesign = async (designId: string, design: ExportedDesign) => {
     ),
   ]);
 
+  // Get Firebase ID token for authentication
+  const idToken = user.value ? await user.value.getIdToken() : null;
+
   await $fetch("/api/designs", {
     method: "POST",
+    headers: idToken ? { Authorization: `Bearer ${idToken}` } : {},
     body: {
       designId,
       templateId: design.templateId,