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/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>