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>

internal/web/resources.go

@@ -14,6 +14,10 @@ import (
 // are safe as both a URL path segment and a filesystem directory name.
 var resourceNameRe = regexp.MustCompile(`^[a-zA-Z0-9_-]{1,64}$`)
 
+// hexColorRe validates the 6-digit hex color format produced by an HTML
+// <input type="color">, e.g. "#3b82f6".
+var hexColorRe = regexp.MustCompile(`^#[0-9a-fA-F]{6}$`)
+
 // handleCalendarResource handles POST (create) and DELETE (remove) for
 // the current user's own calendars, mounted at /resources/calendar.
 func (s *Server) handleCalendarResource(w http.ResponseWriter, r *http.Request) {
@@ -24,6 +28,47 @@ func (s *Server) handleAddressBookResource(w http.ResponseWriter, r *http.Reques
 	s.handleResource(w, r, "addressbook")
 }
 
+// handleCalendarColor updates the color of one of the current user's own
+// calendars, mounted at /resources/calendar/color. It re-renders just
+// that calendar's card (not the whole list), since the set of cards
+// doesn't change.
+func (s *Server) handleCalendarColor(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPost {
+		w.Header().Set("Allow", "POST")
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	username := userFromContext(r.Context())
+	if err := r.ParseForm(); err != nil {
+		http.Error(w, "invalid form", http.StatusBadRequest)
+		return
+	}
+	name := strings.TrimSpace(r.PostForm.Get("name"))
+	color := strings.TrimSpace(r.PostForm.Get("color"))
+	if !resourceNameRe.MatchString(name) {
+		http.Error(w, "name must be 1-64 letters, digits, '-' or '_'", http.StatusBadRequest)
+		return
+	}
+	if color != "" && !hexColorRe.MatchString(color) {
+		http.Error(w, "color must be a hex value like #3b82f6", http.StatusBadRequest)
+		return
+	}
+	if err := s.dbase.SetCalendarColor(username, name, color); err != nil && err != db.ErrResourceNotFound {
+		s.logger.Error("setting calendar color", "error", err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+
+	card, err := s.resourceCardFor(username, "calendar", name)
+	if err != nil {
+		s.logger.Error("rendering resource card", "error", err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	_ = templates.ResourceCardView(card).Render(r.Context(), w)
+}
+
 func (s *Server) handleResource(w http.ResponseWriter, r *http.Request, kind string) {
 	username := userFromContext(r.Context())
 
@@ -50,7 +95,12 @@ func (s *Server) handleResource(w http.ResponseWriter, r *http.Request, kind str
 	case http.MethodPost:
 		var err error
 		if kind == "calendar" {
-			err = s.dbase.CreateCalendar(username, name)
+			color := strings.TrimSpace(r.PostForm.Get("color"))
+			if color != "" && !hexColorRe.MatchString(color) {
+				http.Error(w, "color must be a hex value like #3b82f6", http.StatusBadRequest)
+				return
+			}
+			err = s.dbase.CreateCalendarWithColor(username, name, color)
 		} else {
 			err = s.dbase.CreateAddressBook(username, name)
 		}