refactor: extract date.js, event-factory.js, split.js; use shared date helpers in recur/organize

- 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

src/lib/recur.js

@@ -2,6 +2,7 @@
  * Recurrence grid expansion - port of ical_editor.py's occurrence generation.
  * occurrences are computed on-demand from exdates, never stored on the model.
  */
+import { toISODate, toDate, addDays } from './date.js'
 
 /** Interval in days for a recurring event. */
 export function intervalDays(ev) {
@@ -11,12 +12,7 @@ export function intervalDays(ev) {
   return freq === 'WEEKLY' ? 7 * step : step
 }
 
-function toISO(date) {
-  const y = date.getFullYear()
-  const m = String(date.getMonth() + 1).padStart(2, '0')
-  const d = String(date.getDate()).padStart(2, '0')
-  return `${y}-${m}-${d}`
-}
+const MAX_OCCURRENCES = 500
 
 /**
  * Expand a recurring event into an occurrence grid.
@@ -33,20 +29,18 @@ export function expandOccurrences(ev) {
     return [{ date: ev.dtstartDate, skipped: false }]
   }
 
-  const start = new Date(ev.dtstartDate + 'T00:00:00')
   const until = ev.rrule.untilDate
-    ? new Date(ev.rrule.untilDate + 'T00:00:00')
-    : new Date(start.getTime() + 365 * 24 * 3600 * 1000) // default +1 year
+    ? toDate(ev.rrule.untilDate)
+    : new Date(toDate(ev.dtstartDate).getTime() + 365 * 24 * 3600 * 1000)
 
   const exdateSet = new Set(ev.exdates || [])
   const occs = []
-  const cur = new Date(start)
+  let cur = ev.dtstartDate
   let count = 0
 
-  while (cur <= until && count < 500) {
-    const iso = toISO(cur)
-    occs.push({ date: iso, skipped: exdateSet.has(iso) })
-    cur.setDate(cur.getDate() + step)
+  while (cur <= toISODate(until) && count < MAX_OCCURRENCES) {
+    occs.push({ date: cur, skipped: exdateSet.has(cur) })
+    cur = addDays(cur, step)
     count++
   }