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>

internal/web/templates/layout.templ

@@ -18,6 +18,7 @@ templ Layout(title string, username string) {
 						if username != "" {
 							<a href="/web/files/" class="text-sm text-gray-600 hover:text-indigo-600">Files</a>
 							<a href="/web/contacts" class="text-sm text-gray-600 hover:text-indigo-600">Contacts</a>
+							<a href="/web/calendar" class="text-sm text-gray-600 hover:text-indigo-600">Calendar</a>
 						}
 					</div>
 					if username != "" {