Form validation for event create/update (#218)

* refactor: migrate EventModal to use shared EventFormFields component

* feat: add form validation for event create/update

- Validate title is required
- Validate end datetime must be after start datetime
- Disable save button when form is invalid
- Show error messages below invalid fields with red highlight (MUI)
- Title validation in create mode only shows error after field is touched
- Track touched state to avoid showing errors on initial load

* feat: improve event modal UX with auto-focus and default times

- Auto-focus title field when opening create/update event modal
- Set default datetime when creating event without selecting range:
  * Start time = current time + 1 hour (rounded to :00)
  * End time = start time + 1 hour
- Applied duplicate event to display datetime from original event
- Fix logic to detect empty event object vs valid event with uid

* fix: improve validation UX and add required field indicator

- Only show validation errors when modal is open (isOpen check)
- Add red asterisk (*) to Title label to indicate required field
- Prevent validation error flash when closing modal
- Fix TypeScript errors in EventUpdateModal
- Keep validation behavior: only show error after user types and deletes

* feat: auto-focus title field when toggling between normal and extended mode in event modal


* update: only validation when click on save button

* fix: prevent form data loss and validation errors in event modals

Fixes:
1. Form fields no longer clear when changing start/end dates
2. Extended mode stays active when modifying dates
3. No validation errors shown after successful save

src/components/Calendar/handlers/eventHandlers.ts

@@ -16,7 +16,7 @@ import { getEvent } from "../../../features/Events/EventApi";
 import { refreshCalendars } from "../../Event/utils/eventUtils";
 import { updateTempCalendar } from "../utils/calendarUtils";
 import { User } from "../../Attendees/PeopleSearch";
-import { formatLocalDateTime } from "../../../features/Events/EventModal";
+import { formatLocalDateTime } from "../../Event/EventFormFields";
 
 export interface EventHandlersProps {
   setSelectedRange: (range: DateSelectArg | null) => void;