fix: use JSON clone instead of structuredClone for Vue reactive proxies

structuredClone cannot clone Vue reactive Proxy objects (DataCloneError).
EventModel contains only plain data, so JSON round-trip is safe and
lossless. Verified: organize, toggle, undo, download all work correctly.

src/lib/event-factory.js

@@ -30,12 +30,13 @@ export function createEvent(overrides = {}) {
 }
 
 /**
- * Deep-clone an event via structuredClone (available in modern browsers
- * and Node 17+). Falls back to JSON round-trip for older environments.
+ * Deep-clone an event (or any plain-data object).
+ *
+ * Uses JSON round-trip rather than structuredClone because events may be
+ * wrapped in Vue reactive proxies, which structuredClone cannot handle.
+ * EventModel contains only plain data (strings, arrays, nested objects),
+ * so JSON cloning is safe and lossless here.
  */
 export function cloneEvent(ev) {
-  if (typeof structuredClone === 'function') {
-    return structuredClone(ev)
-  }
   return JSON.parse(JSON.stringify(ev))
 }