feat(editor): add drag handle plugin

Expose headless block metadata and move commands for drag handles.

Compose the docs playground handle, insertion line, and dragged-block overlay through consumer-owned UI.

Author: cdskill

apps/docs/src/app/docs/docs-nav.ts

@@ -69,6 +69,7 @@ export const DOCS_NAV: readonly DocsNavGroup[] = [
     items: [
       { title: 'Slash Commands', href: '/docs/slash-command' },
       { title: 'Mention', href: '/docs/mention' },
+      { title: 'Drag Handle', href: '/docs/drag-handle' },
       { title: 'History (Undo/Redo)', href: '/docs/history' },
       { title: 'Paste Rules', href: '/docs/paste-rules' },
       { title: 'Placeholder', href: '/docs/placeholder' },

apps/docs/src/app/pages/(home).page.ts

@@ -16,7 +16,7 @@ import { Playground } from '../playground/playground';
 
       <section
         id="playground"
-        class="mx-auto max-w-5xl px-4 pb-24 sm:px-6"
+        class="mx-auto max-w-6xl px-4 pb-24 sm:px-6"
         aria-label="Live editor playground"
       >
         @defer (on viewport) {