Add calendar color support (DAVx5 calendar-color)

Calendars can now have a color (hex, e.g. #3b82f6) that DAVx5 and other
CalDAV clients pick up via the Apple/dav4jvm calendar-color property.

- db: add calendars.color column with migration for existing DBs;
  CreateCalendarWithColor, SetCalendarColor, GetCalendarColor;
  ListCalendars now returns []Calendar{Name, Color} instead of []string
- caldav: since go-webdav's caldav.Backend interface has no extension
  point for vendor properties, wrap the handler with a response-rewriting
  middleware that injects <calendar-color xmlns="http://apple.com/ns/ical/">
  into PROPFIND responses for calendars that have a color set
- web: color picker on the "New calendar" form and an inline color swatch/
  picker on each calendar card (calendars only, not address books)
- nidusctl: `calendar create --color` flag and a new `calendar color`
  subcommand; `calendar list` now also prints the color if set

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

cmd/server/main.go

@@ -82,8 +82,8 @@ func main() {
 			continue
 		}
 		for _, cal := range cals {
-			if err := st.EnsureCollection(user.Username, "cal-"+cal); err != nil {
-				logger.Warn("creating calendar collection", "user", user.Username, "cal", cal, "error", err)
+			if err := st.EnsureCollection(user.Username, "cal-"+cal.Name); err != nil {
+				logger.Warn("creating calendar collection", "user", user.Username, "cal", cal.Name, "error", err)
 			}
 		}
 		books, err := dbase.ListAddressBooks(user.Username)