fix(calendar): use date-only strings for all-day events to prevent timezone shift

UTC midnight passed to FullCalendar's timeZone:'local' was being
converted to local time, shifting all-day events back by 1+ days for
users in UTC-X zones. The edit form had the same bug via new Date().

Fix: pass YYYY-MM-DD slices (UTC date) for all-day events in both
toFcEvent and EventSlideOver resetForm, bypassing timezone conversion.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/CalendarView.vue

@@ -39,11 +39,13 @@ function closeSlideOver() {
 const eventCache = new Map<number, EventEntry>();
 
 function toFcEvent(e: EventEntry) {
+  // For all-day events pass date-only strings so FullCalendar never shifts
+  // the date through timezone conversion (UTC midnight → previous day in UTC-X).
   return {
     id: String(e.id),
     title: e.title,
-    start: e.start_dt,
-    end: e.end_dt ?? undefined,
+    start: e.all_day ? e.start_dt.slice(0, 10) : e.start_dt,
+    end: e.all_day ? (e.end_dt?.slice(0, 10) ?? undefined) : (e.end_dt ?? undefined),
     allDay: e.all_day,
     backgroundColor: e.color || undefined,
     borderColor: e.color || undefined,