Fix: Preserve timezone used in create and update modal (#301)

- Fix timezone field display in event modals to show original timezone from server
- Fix formatDateToICal to use moment.utc(date).tz(timezone) for correct UTC instant preservation
- Fix parseCalendarEvent to convert event.start/end to ISO UTC strings after parsing
- Fix eventToFullCalendarFormat to ensure event times are ISO UTC before passing to FullCalendar
- Fix getEvent to normalize event.start/end to ISO UTC strings
- Ensure event.timezone is always set (defaults to Etc/UTC if not detected)
- Update eventUtils.test.ts to match new timezone conversion logic
- Fix event time shift issue on calendar grid display

This ensures:
- Events display in correct time slots on calendar grid
- Event times are correctly converted between timezones
- Original timezone from server is preserved and displayed
- All event times are stored as ISO UTC strings internally

* refactor: eliminate datetime format detection code duplication

- Add constants for datetime format strings and magic number (DATETIME_WITH_SECONDS_LENGTH, DATETIME_FORMAT_WITH_SECONDS, DATETIME_FORMAT_WITHOUT_SECONDS)
- Create shared detectDateTimeFormat() function to replace duplicated format detection logic
- Refactor convertFormDateTimeToISO, convertEventDateTimeToISO, and convertDateTimeStringToISO to use shared helper
- Add console.warn logging when invalid datetime is encountered (addresses silent fallback issue)
- Add comprehensive test coverage for dateTimeHelpers with full test suite

src/components/Event/utils/dateTimeHelpers.ts

@@ -1,7 +1,24 @@
+import moment from "moment-timezone";
+
 /**
  * Helper functions for date/time string manipulation
  */
 
+export const DATETIME_WITH_SECONDS_LENGTH = 19;
+export const DATETIME_FORMAT_WITH_SECONDS = "YYYY-MM-DDTHH:mm:ss";
+export const DATETIME_FORMAT_WITHOUT_SECONDS = "YYYY-MM-DDTHH:mm";
+
+/**
+ * Detect datetime format based on string length
+ * @param datetime - Datetime string to analyze
+ * @returns Format string for moment parsing
+ */
+export function detectDateTimeFormat(datetime: string): string {
+  return datetime.length >= DATETIME_WITH_SECONDS_LENGTH
+    ? DATETIME_FORMAT_WITH_SECONDS
+    : DATETIME_FORMAT_WITHOUT_SECONDS;
+}
+
 /**
  * Split datetime string (YYYY-MM-DDTHH:mm) into date and time parts
  * @param datetime - ISO datetime string
@@ -30,3 +47,24 @@ export function combineDateTime(date: string, time: string): string {
   if (!time) return date; // Date only for all-day
   return `${date}T${time}`;
 }
+
+/**
+ * Convert a local form datetime string to ISO string in a specific timezone
+ * Assumes input format YYYY-MM-DDTHH:mm (24h)
+ */
+export function convertFormDateTimeToISO(
+  datetime: string,
+  timezone: string
+): string {
+  if (!datetime) return "";
+  const tz = timezone || "Etc/UTC";
+  const format = detectDateTimeFormat(datetime);
+  const momentDate = moment.tz(datetime, format, tz);
+  if (!momentDate.isValid()) {
+    console.warn(
+      `[convertFormDateTimeToISO] Invalid datetime: "${datetime}" with format "${format}" in timezone "${tz}"`
+    );
+    return "";
+  }
+  return momentDate.toDate().toISOString();
+}