Cross-page copy/paste support

Problem/Motivation

The Display Builder clipboard currently stores only a node ID reference in browser localStorage, scoped by builder instance entity ID:
localStorage key: "Drupal.{instance_id}"
value: {"copy": {"id": "abc123def456", "title": "My Component"}}
Copy is a purely client-side operation (no server request). When pasting, the server fetches the full component subtree by node ID from the current builder instance via getNode(). This means the clipboard is meaningless outside the current builder instance — the stored node ID refers to nothing on a different page.

Steps to reproduce

1. Edit a node translation (e.g. English) in Display Builder
2. Copy a component (right-click → Copy)
3. Navigate to another builder instance (different entity, or different language of the same entity that has a separate builder)
4. Right-click → Paste → nothing happens (paste is disabled because copyInstance.id is null under the new instance's localStorage namespace)

Proposed resolution

1. New API endpoint POST /api/display-builder/{instance}/_node/{node_id}/clipboard — returns the full subtree via SourceTree::getNode() as JSON
2. Copy flow: HTMX POST to the new endpoint → server returns JSON → JS stores in global Drupal.display_builder_clipboard (namespaced by project, not by instance)
3. Paste flow: JS reads global clipboard → passes source_data in request body → server uses it if present, falls back to getNode() for backward compatibility
4. Old clipboard format (just {id, title}) degrades gracefully: paste is disabled when no data field exists

Remaining tasks

User interface changes

API changes

Data model changes