feat(builder): implement Ctrl+C/V copy-paste shortcuts

The context menu advertised Ctrl+C/Ctrl+V hints but useKeyboardShortcuts
only handled undo/redo/delete/duplicate/escape -- pressing them did
nothing. Add a tiny shared module-level clipboard (src/hooks/clipboard.ts)
used by both the keyboard hook and the context menu so copying via one
entry point and pasting via the other stay consistent.

Ctrl/Cmd+C stores the selected node id (skipping ROOT). Ctrl/Cmd+V
inserts a copy as a sibling of the current selection via
regenerateTreeIds, mirroring the existing context-menu paste behavior.
Both respect the existing "disabled while typing" guard.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/panels/context-menu/ContextMenu.tsx

@@ -4,6 +4,7 @@ import { findDeletableTarget } from '../../utils/craft-helpers';
 import { useSitesmithModal } from '../../state/SitesmithContext';
 import { buildSitesmithTarget } from '../../utils/sitesmith-target';
 import { regenerateTreeIds } from '../../utils/craft-tree';
+import { getClipboardNodeId, setClipboardNodeId } from '../../hooks/clipboard';
 
 interface ContextMenuProps {
   visible: boolean;
@@ -32,7 +33,6 @@ export const ContextMenu: React.FC<ContextMenuProps> = ({
   const { actions, query } = useEditor();
   const { open: openSitesmith } = useSitesmithModal();
   const menuRef = useRef<HTMLDivElement>(null);
-  const clipboardRef = useRef<string | null>(null);
 
   // Close on click outside
   useEffect(() => {
@@ -80,7 +80,7 @@ export const ContextMenu: React.FC<ContextMenuProps> = ({
   const copyNode = useCallback(() => {
     if (!nodeId || nodeId === 'ROOT') return;
     try {
-      clipboardRef.current = nodeId;
+      setClipboardNodeId(nodeId);
     } catch (e) {
       console.error('Copy failed:', e);
     }
@@ -88,7 +88,7 @@ export const ContextMenu: React.FC<ContextMenuProps> = ({
   }, [nodeId, onClose]);
 
   const pasteNode = useCallback(() => {
-    const sourceId = clipboardRef.current;
+    const sourceId = getClipboardNodeId();
     if (!sourceId) {
       onClose();
       return;
@@ -210,7 +210,7 @@ export const ContextMenu: React.FC<ContextMenuProps> = ({
       label: 'Paste',
       shortcut: 'Ctrl+V',
       action: pasteNode,
-      disabled: !clipboardRef.current,
+      disabled: !getClipboardNodeId(),
       dividerAfter: true,
     },
     {