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/hooks/useKeyboardShortcuts.ts

@@ -2,6 +2,7 @@ import { useEffect } from 'react';
 import { useEditor } from '@craftjs/core';
 import { findDeletableTarget } from '../utils/craft-helpers';
 import { regenerateTreeIds } from '../utils/craft-tree';
+import { getClipboardNodeId, setClipboardNodeId } from './clipboard';
 
 function isInputFocused(): boolean {
   const el = document.activeElement;
@@ -85,6 +86,45 @@ export function useKeyboardShortcuts() {
         return;
       }
 
+      // Ctrl+C: copy selected node id to the shared clipboard
+      if (ctrl && (e.key === 'c' || e.key === 'C')) {
+        e.preventDefault();
+        try {
+          const selected = query.getEvent('selected').all();
+          if (selected.length > 0 && selected[0] !== 'ROOT') {
+            setClipboardNodeId(selected[0]);
+          }
+        } catch (err) {
+          console.error('Copy failed:', err);
+        }
+        return;
+      }
+
+      // Ctrl+V: paste the clipboard node as a sibling of the current selection
+      if (ctrl && (e.key === 'v' || e.key === 'V')) {
+        e.preventDefault();
+        try {
+          const sourceId = getClipboardNodeId();
+          if (!sourceId || !query.node(sourceId).get()) return;
+
+          const selected = query.getEvent('selected').all();
+          if (selected.length === 0) return;
+          const selectedId = selected[0];
+
+          let targetParent = 'ROOT';
+          if (selectedId !== 'ROOT') {
+            const node = query.node(selectedId).get();
+            targetParent = node?.data?.parent || 'ROOT';
+          }
+
+          const tree = regenerateTreeIds(query.node(sourceId).toNodeTree());
+          actions.addNodeTree(tree, targetParent);
+        } catch (err) {
+          console.error('Paste failed:', err);
+        }
+        return;
+      }
+
       // Escape: deselect all
       if (e.key === 'Escape') {
         e.preventDefault();