refactor: high-cohesion low-coupling cleanup across store and components

ical-io.js:
- Import pad2/parseISODate from date.js (remove local duplicates)
- Rename ICAL.Time helpers to icalTimeTo* for clarity

useCalendar.js:
- Delegate addEvent to createEvent() factory
- Delegate splitEvent to splitRecurringEvent() pure function
- Use cloneEvent() instead of JSON.parse(JSON.stringify())
- Add selectEvent() action (components no longer mutate state.selectedUid directly)

Components:
- EventList: use selectEvent action + toDate() from date.js
- HeaderBar: destructure loadFile at top level (was re-calling useCalendar in handler)
- EventDetail: import DOW from weekday.js (was inlining the array)
- OccurrenceGrid: use toDate() from date.js

src/composables/useCalendar.js

@@ -1,6 +1,8 @@
 import { reactive, computed } from 'vue'
 import { parseICS, serializeICS } from '../lib/ical-io.js'
 import { organize as organizeEvents } from '../lib/organize.js'
+import { createEvent, cloneEvent } from '../lib/event-factory.js'
+import { splitRecurringEvent } from '../lib/split.js'
 
 const MAX_HISTORY = 50
 
@@ -20,8 +22,8 @@ const state = reactive({
 // ------------------------------------------------------------------ //
 function snapshot() {
   return {
-    meta: state.meta ? JSON.parse(JSON.stringify(state.meta)) : null,
-    events: JSON.parse(JSON.stringify(state.events)),
+    meta: state.meta ? cloneEvent(state.meta) : null,
+    events: state.events.map(cloneEvent),
     selectedUid: state.selectedUid,
   }
 }
@@ -33,8 +35,8 @@ function pushHistory() {
 }
 
 function restore(snap) {
-  state.meta = snap.meta ? JSON.parse(JSON.stringify(snap.meta)) : null
-  state.events = JSON.parse(JSON.stringify(snap.events))
+  state.meta = snap.meta ? cloneEvent(snap.meta) : null
+  state.events = snap.events.map(cloneEvent)
   state.selectedUid = snap.selectedUid
 }
 
@@ -54,6 +56,10 @@ function loadFile(file) {
   })
 }
 
+function selectEvent(uid) {
+  state.selectedUid = uid
+}
+
 function organize() {
   if (state.events.length === 0) return
   pushHistory()
@@ -81,23 +87,7 @@ function deleteEvent(uid) {
 
 function addEvent() {
   pushHistory()
-  const today = new Date()
-  const y = today.getFullYear()
-  const m = String(today.getMonth() + 1).padStart(2, '0')
-  const d = String(today.getDate()).padStart(2, '0')
-  const ev = {
-    uid: crypto.randomUUID(),
-    summary: '新日程',
-    location: '',
-    description: '',
-    dtstartDate: `${y}-${m}-${d}`,
-    dtstartTime: '09:00',
-    dtendTime: '10:00',
-    tzid: 'UTC',
-    rrule: null,
-    exdates: [],
-    _raw: { dtstamp: new Date().toISOString().replace(/[-:]/g, '').split('.')[0] + 'Z' },
-  }
+  const ev = createEvent()
   state.events.push(ev)
   state.selectedUid = ev.uid
   state.dirty = true
@@ -107,34 +97,11 @@ function splitEvent(uid, splitDate) {
   const ev = state.events.find((e) => e.uid === uid)
   if (!ev || !ev.rrule) return
 
-  // Snap splitDate to the next grid point >= splitDate
-  const step = ev.rrule.freq === 'WEEKLY' ? 7 * (ev.rrule.interval || 1) : (ev.rrule.interval || 1)
-  const start = new Date(ev.dtstartDate + 'T00:00:00')
-  let snap = new Date(splitDate + 'T00:00:00')
-  // Advance snap to the next grid point on or after splitDate
-  while (snap < start || ((snap - start) / (24 * 3600 * 1000)) % step !== 0) {
-    if (snap < start) { snap = new Date(start); break }
-    snap.setDate(snap.getDate() + 1)
-    if (snap > new Date(ev.rrule.untilDate + 'T00:00:00')) return // out of range
-  }
-  const snapStr = `${snap.getFullYear()}-${String(snap.getMonth() + 1).padStart(2, '0')}-${String(snap.getDate()).padStart(2, '0')}`
-
-  // Part A: rrule.untilDate = day before snap (previous grid point)
-  const prevGrid = new Date(snap)
-  prevGrid.setDate(prevGrid.getDate() - step)
-  const prevStr = `${prevGrid.getFullYear()}-${String(prevGrid.getMonth() + 1).padStart(2, '0')}-${String(prevGrid.getDate()).padStart(2, '0')}`
+  const parts = splitRecurringEvent(ev, splitDate)
+  if (!parts) return
 
   pushHistory()
-
-  const partA = JSON.parse(JSON.stringify(ev))
-  partA.rrule = { ...ev.rrule, untilDate: prevStr }
-  partA.exdates = ev.exdates.filter((d) => d < splitDate)
-
-  const partB = JSON.parse(JSON.stringify(ev))
-  partB.uid = crypto.randomUUID()
-  partB.dtstartDate = snapStr
-  partB.exdates = ev.exdates.filter((d) => d >= splitDate)
-
+  const [partA, partB] = parts
   const idx = state.events.findIndex((e) => e.uid === uid)
   state.events.splice(idx, 1, partA, partB)
   state.selectedUid = partB.uid
@@ -199,6 +166,7 @@ export function useCalendar() {
     isLoaded,
     stats,
     loadFile,
+    selectEvent,
     organize,
     updateEvent,
     deleteEvent,