Make birthdays calendar color configurable and expose it via CalDAV

- Add a per-user birthday_calendars table (color, cascade-deletes with
  the user) and GetBirthdayCalendarColor/SetBirthdayCalendarColor in
  internal/db, plus a reservedCalendarNames guard ("birthdays") in
  CreateCalendarWithColor so no real calendar can collide with the
  synthetic one, whether created via the web UI, nidusctl, or CalDAV
  MKCALENDAR.
- Add a "Birthdays" virtual resource card to the dashboard (color
  picker only, no delete/share controls) backed by a new
  ResourceCard.Virtual flag and POST /web/resources/birthdays/color
  handler.
- Extract the birthday-parsing/generation logic shared by the web
  calendar view and CalDAV into internal/birthdays (ParseBirthday,
  Collect, OccurrenceDate, Summary) instead of duplicating it.
- Expose the Birthdays calendar over real CalDAV in
  internal/caldav/backend.go + birthdays.go: it's always listed for
  every user, generates one VEVENT per (contact, year) for a rolling
  window (current year -2..+8) with "🎂 Name (Age)" titles, is
  read-only (Put/Delete/DeleteCalendar all return 403), and its
  Apple/DAVx5 calendar-color is injected from the same per-user
  setting used by the dashboard/web view.

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

internal/web/server.go

@@ -44,6 +44,7 @@ func (s *Server) Handler(staticFS http.FileSystem) http.Handler {
 	mux.HandleFunc("/shares/addressbook", s.requireLogin(s.handleAddressBookShare))
 	mux.HandleFunc("/resources/calendar", s.requireLogin(s.handleCalendarResource))
 	mux.HandleFunc("/resources/calendar/color", s.requireLogin(s.handleCalendarColor))
+	mux.HandleFunc("/resources/birthdays/color", s.requireLogin(s.handleBirthdayColor))
 	mux.HandleFunc("/resources/addressbook", s.requireLogin(s.handleAddressBookResource))
 	mux.HandleFunc("/files/{path...}", s.requireLogin(s.handleFiles))
 	mux.HandleFunc("/contacts", s.requireLogin(s.handleContactsHome))