feat(ui): replace end date field with '...' button toggle

- Hide end date by default; reveal via '...' button in both modes
- Keep state/validation/API logic unchanged
- Normalize labels to use MUI TextField labels across modes
- Align button with fields; constrain widths for expanded mode

src/components/Event/utils/dateRules.ts

@@ -0,0 +1,62 @@
+// Date rules helpers to normalize end date/time behavior
+
+import { combineDateTime } from "./dateTimeHelpers";
+
+/** Adds a number of days to a YYYY-MM-DD string and returns YYYY-MM-DD */
+export function addDays(dateStr: string, days: number): string {
+  const d = new Date(dateStr);
+  d.setDate(d.getDate() + days);
+  return d.toISOString().split("T")[0];
+}
+
+/**
+ * Compute endDate (YYYY-MM-DD) when startDate changes
+ * - If all-day: endDate = startDate + 1 day
+ * - Else: endDate = startDate
+ */
+export function getEndDateForStartChange(
+  startDate: string,
+  isAllDay: boolean
+): string {
+  if (!startDate) return "";
+  return isAllDay ? addDays(startDate, 1) : startDate;
+}
+
+/**
+ * Compute new start/end (ISO-like strings) when toggling all-day
+ * - nextAllDay = true:
+ *   - If fromAllDaySlot: endDate = startDate
+ *   - Else: endDate = startDate + 1 day
+ *   - Times are cleared (date-only)
+ * - nextAllDay = false:
+ *   - end restored from originalEndDate if any, else previousEndDate, else start
+ */
+export function getEndDateForToggle(params: {
+  nextAllDay: boolean;
+  fromAllDaySlot?: boolean;
+  startDate: string;
+  previousEndDate: string;
+  originalEndDate?: string;
+}): string {
+  const {
+    nextAllDay,
+    fromAllDaySlot,
+    startDate,
+    previousEndDate,
+    originalEndDate,
+  } = params;
+  if (nextAllDay) {
+    return fromAllDaySlot ? startDate : addDays(startDate, 1);
+  }
+  return originalEndDate || previousEndDate || startDate;
+}
+
+/** Utility to combine date with a fallback time (HH:mm) safely */
+export function combineWithFallback(
+  dateStr: string,
+  timeHHmm: string | undefined,
+  fallbackTime: string
+): string {
+  const time = timeHHmm && timeHHmm.trim() ? timeHHmm : fallbackTime;
+  return combineDateTime(dateStr, time);
+}