Warn before closing page if edits are not saved

[Copilot]

Users could accidentally lose unsaved calendar edits by closing the tab or navigating away — no confirmation was shown.

Changes

• isDirty flag — module-level boolean, set to true on any edit, reset to false on submit
• Edit sources that mark dirty:
  • eventClick — slot availability toggle or span shift-click/maybe-toggle
  • calSelect — new span drawn
  • eventDrop / eventResize (new FullCalendar callbacks) — span dragged or resized
  • Response textarea input event
• beforeunload listener — triggers browser's native "Leave site?" dialog when isDirty:

  window.addEventListener('beforeunload', (e) => {
    if (isDirty) {
      e.preventDefault();
      e.returnValue = ''; // required for cross-browser support
    }
  });

• onSubmit resets isDirty = false before form submission so the dialog doesn't fire on a clean submit