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.
- date.js: single source of truth for ISO date formatting/arithmetic
(toISODate, parseISODate, addDays, daysBetween, todayISO, nowCompactTimestamp)
- event-factory.js: createEvent/cloneEvent (structuredClone with JSON fallback)
- split.js: splitRecurringEvent pure function using intervalDays from recur.js
- recur.js: replaced local toISO() and inline Date math with date.js helpers
- organize.js: replaced inline date formatting and gap math with date.js helpers,
extracted buildGrid() helper and byDate comparator
