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.ts

@@ -0,0 +1,23 @@
+/**
+ * Tiny shared clipboard for canvas node copy/paste.
+ *
+ * Both the context menu (right-click Copy/Paste) and the keyboard shortcuts
+ * hook (Ctrl/Cmd+C / Ctrl/Cmd+V) read and write this single module-level
+ * store, so copying a node via one entry point and pasting via the other
+ * behaves consistently instead of each maintaining its own clipboard.
+ *
+ * Deliberately not React state -- nothing in the UI needs to re-render
+ * reactively when the clipboard changes; consumers just read the current
+ * value at the moment they need it (on paste, or when a menu opens).
+ */
+let clipboardNodeId: string | null = null;
+
+/** Returns the id of the node currently on the clipboard, or null if empty. */
+export function getClipboardNodeId(): string | null {
+  return clipboardNodeId;
+}
+
+/** Sets (or clears, with `null`) the node id on the clipboard. */
+export function setClipboardNodeId(nodeId: string | null): void {
+  clipboardNodeId = nodeId;
+}