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/components/HeaderBar.vue

@@ -2,7 +2,7 @@
 import { useCalendar } from '../composables/useCalendar.js'
 import { ref } from 'vue'
 
-const { state, canUndo, canRedo, organize, undo, redo, serialize, markSaved } = useCalendar()
+const { state, canUndo, canRedo, organize, undo, redo, serialize, markSaved, loadFile } = useCalendar()
 const fileInput = ref(null)
 
 function onOrganize() {
@@ -19,7 +19,6 @@ function onImport() {
 function onFilePicked(e) {
   const file = e.target.files[0]
   if (!file) return
-  const { loadFile } = useCalendar()
   loadFile(file).catch((err) => alert('解析失败：' + (err.message || err)))
   e.target.value = ''
 }