fix: make folk-rapp header a drag handle so shapes can be grabbed

folk-rapp used class "rapp-header" but the drag detection in
folk-shape checks for ".header" or "[data-drag]". Added both so
the rapp header works as a drag handle like every other shape type.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/folk-rapp.ts

@@ -358,7 +358,7 @@ export class FolkRApp extends FolkShape {
 
 		const wrapper = document.createElement("div");
 		wrapper.innerHTML = html`
-			<div class="rapp-header" style="--rapp-color: ${headerColor}">
+			<div class="header rapp-header" data-drag style="--rapp-color: ${headerColor}">
 				<div class="rapp-header-left">
 					<span class="rapp-badge">${headerBadge}</span>
 					<span class="rapp-name">${headerName}</span>