Add web UI for CalDAV calendars (web/calendar)

- List of the user's own calendars (web/calendar)
- Month view per calendar with a Monday-first 6-week grid, showing
  all-day and timed events, colored dot matching the calendar's color
- Navigation between months (prev/next/today) via query params
- Create/edit/delete events: title, description, location, all-day
  toggle, start/end date+time
- Clicking a day cell prefills a new all-day event on that date; the
  "New event" button defaults to a one-hour slot starting next full hour
- Deleting an event or invalid time ranges (end before/equal start) are
  validated with inline error messages
- Verified round-trip with the real CalDAV protocol handler (events
  created via the web UI are correctly visible to a REPORT query)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

web/static/calendar.js

@@ -0,0 +1,12 @@
+"use strict";
+// Event form behavior: hide/show the start/end time fields depending on
+// whether the "All-day event" checkbox is checked.
+(function initEventForm() {
+    const allDay = document.getElementById("all-day");
+    const startTimeField = document.getElementById("start-time-field");
+    const endTimeField = document.getElementById("end-time-field");
+    allDay?.addEventListener("change", () => {
+        startTimeField?.classList.toggle("hidden", allDay.checked);
+        endTimeField?.classList.toggle("hidden", allDay.checked);
+    });
+})();