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/lib/ical-io.js

@@ -1,4 +1,5 @@
 import ICAL from 'ical.js'
+import { pad2, parseISODate } from './date.js'
 
 // ------------------------------------------------------------------ //
 // Timezone registration (ical.js does not bundle IANA zones)
@@ -13,17 +14,14 @@ function registerTimezones(vcalendar) {
 }
 
 // ------------------------------------------------------------------ //
-// Helpers
+// ICAL.Time <-> plain string helpers
+// (ICAL.Time exposes .year/.month/.day/.hour/.minute directly)
 // ------------------------------------------------------------------ //
-function pad2(n) {
-  return String(n).padStart(2, '0')
-}
-
-function timeToISODate(t) {
+function icalTimeToISODate(t) {
   return `${t.year}-${pad2(t.month)}-${pad2(t.day)}`
 }
 
-function timeToHHMM(t) {
+function icalTimeToHHMM(t) {
   return `${pad2(t.hour)}:${pad2(t.minute)}`
 }
 
@@ -67,7 +65,7 @@ function parseEvent(veventComp) {
   const exdates = []
   for (const prop of veventComp.getAllProperties('exdate')) {
     for (const t of prop.getValues()) {
-      exdates.push(timeToISODate(t))
+      exdates.push(icalTimeToISODate(t))
     }
   }
 
@@ -83,9 +81,9 @@ function parseEvent(veventComp) {
     summary: veventComp.getFirstPropertyValue('summary') || '',
     location: veventComp.getFirstPropertyValue('location') || '',
     description: veventComp.getFirstPropertyValue('description') || '',
-    dtstartDate: timeToISODate(start),
-    dtstartTime: timeToHHMM(start),
-    dtendTime: timeToHHMM(end),
+    dtstartDate: icalTimeToISODate(start),
+    dtstartTime: icalTimeToHHMM(start),
+    dtendTime: icalTimeToHHMM(end),
     tzid,
     rrule,
     exdates,
@@ -125,11 +123,6 @@ export function parseICS(text) {
 // ------------------------------------------------------------------ //
 // Helpers for serialization
 // ------------------------------------------------------------------ //
-function parseISODate(s) {
-  const [y, m, d] = s.split('-').map(Number)
-  return { year: y, month: m, day: d }
-}
-
 function parseHHMM(s) {
   const [h, m] = s.split(':').map(Number)
   return { hour: h, minute: m }