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/date.js

@@ -0,0 +1,59 @@
+/**
+ * Centralized date-string utilities.
+ *
+ * All date values in the app use the 'YYYY-MM-DD' (wall-clock) format.
+ * These helpers are the single source of truth for formatting and
+ * arithmetic on that format, replacing ad-hoc inline code that was
+ * duplicated across organize.js, recur.js, useCalendar.js, and ical-io.js.
+ */
+
+/** Pad a number to 2 digits with leading zero. */
+export function pad2(n) {
+  return String(n).padStart(2, '0')
+}
+
+/** Convert a JS Date to 'YYYY-MM-DD' (local wall-clock). */
+export function toISODate(date) {
+  return `${date.getFullYear()}-${pad2(date.getMonth() + 1)}-${pad2(date.getDate())}`
+}
+
+/** Parse 'YYYY-MM-DD' into { year, month, day } (month is 1-based). */
+export function parseISODate(s) {
+  const [y, m, d] = s.split('-').map(Number)
+  return { year: y, month: m, day: d }
+}
+
+/** Build a local JS Date at midnight from 'YYYY-MM-DD'. */
+export function toDate(isoDate) {
+  return new Date(isoDate + 'T00:00:00')
+}
+
+/** Add `days` days to an ISO date string, return new ISO string. */
+export function addDays(isoDate, days) {
+  const d = toDate(isoDate)
+  d.setDate(d.getDate() + days)
+  return toISODate(d)
+}
+
+/** Whole-day difference between two ISO date strings (b - a). */
+export function daysBetween(isoA, isoB) {
+  return Math.round((toDate(isoB) - toDate(isoA)) / (24 * 3600 * 1000))
+}
+
+/** Today's date as 'YYYY-MM-DD' (local). */
+export function todayISO() {
+  return toISODate(new Date())
+}
+
+/**
+ * Current UTC timestamp in compact RFC 5545 form (e.g. '20260711T085008Z').
+ * Used for DTSTAMP on new events.
+ */
+export function nowCompactTimestamp() {
+  return new Date().toISOString().replace(/[-:]/g, '').split('.')[0] + 'Z'
+}
+
+/** Format a JS Date's time as 'HH:MM'. */
+export function toHHMM(date) {
+  return `${pad2(date.getHours())}:${pad2(date.getMinutes())}`
+}