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/clipboard.test.ts

@@ -0,0 +1,29 @@
+import { describe, test, expect, afterEach } from 'vitest';
+import { getClipboardNodeId, setClipboardNodeId } from './clipboard';
+
+describe('clipboard', () => {
+  afterEach(() => {
+    setClipboardNodeId(null);
+  });
+
+  test('starts empty', () => {
+    expect(getClipboardNodeId()).toBeNull();
+  });
+
+  test('set then get returns the stored node id', () => {
+    setClipboardNodeId('node-123');
+    expect(getClipboardNodeId()).toBe('node-123');
+  });
+
+  test('is a shared module-level store -- overwriting replaces the previous value', () => {
+    setClipboardNodeId('first');
+    setClipboardNodeId('second');
+    expect(getClipboardNodeId()).toBe('second');
+  });
+
+  test('can be cleared back to null', () => {
+    setClipboardNodeId('node-123');
+    setClipboardNodeId(null);
+    expect(getClipboardNodeId()).toBeNull();
+  });
+});