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/EventDetail.vue

@@ -1,7 +1,7 @@
 <script setup>
 import { ref, watch } from 'vue'
 import { useCalendar } from '../composables/useCalendar.js'
-import { DOW_EN } from '../lib/weekday.js'
+import { DOW, DOW_EN } from '../lib/weekday.js'
 import OccurrenceGrid from './OccurrenceGrid.vue'
 
 const { selectedEvent, updateEvent, deleteEvent, splitEvent, toggleOccurrence } = useCalendar()
@@ -142,7 +142,7 @@ function onToggleOcc(date) {
           <label>星期 (BYDAY)</label>
           <select v-model="form.byday">
             <option v-for="(d, i) in DOW_EN" :key="d" :value="d">
-              {{ ['周日','周一','周二','周三','周四','周五','周六'][i] }}
+              {{ DOW[i] }}
             </option>
           </select>
         </div>

src/components/EventList.vue

@@ -1,15 +1,12 @@
 <script setup>
 import { useCalendar } from '../composables/useCalendar.js'
 import { DOW } from '../lib/weekday.js'
+import { toDate } from '../lib/date.js'
 
-const { state, stats, addEvent } = useCalendar()
-
-function selectEvent(uid) {
-  state.selectedUid = uid
-}
+const { state, stats, addEvent, selectEvent } = useCalendar()
 
 function dowFor(ev) {
-  return DOW[new Date(ev.dtstartDate + 'T00:00:00').getDay()]
+  return DOW[toDate(ev.dtstartDate).getDay()]
 }
 </script>
 

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 = ''
 }

src/components/OccurrenceGrid.vue

@@ -1,6 +1,7 @@
 <script setup>
 import { computed } from 'vue'
 import { DOW } from '../lib/weekday.js'
+import { toDate } from '../lib/date.js'
 import { expandOccurrences } from '../lib/recur.js'
 
 const props = defineProps({
@@ -11,7 +12,7 @@ const emit = defineEmits(['toggle'])
 const occurrences = computed(() => expandOccurrences(props.event))
 
 function dowFor(dateStr) {
-  return DOW[new Date(dateStr + 'T00:00:00').getDay()]
+  return DOW[toDate(dateStr).getDay()]
 }
 </script>