Use display name instead of username for shared calendar owners

Add db.DisplayName(username), returning the user's display name if
set, otherwise the raw username. Use it everywhere a shared calendar's
owner is shown: the web calendar legend ("shared by ..."), the "new
event"/edit calendar select label, the dashboard's "Shared with you"
list, and the CalDAV-side description/name-collision disambiguation
("Name (Owner)") shown to CalDAV clients like DAVx5.

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

internal/caldav/backend.go

@@ -382,7 +382,7 @@ func (b *Backend) calendarMeta(owner, realName, localName string) caldav.Calenda
 // their own calendars (see sharedDisplayName) — the Description still
 // always names the real calendar, only the client-visible Name changes.
 func (b *Backend) calendarMetaNamed(owner, realName, localName, displayName string) caldav.Calendar {
-	desc := fmt.Sprintf("%s's %s calendar", owner, realName)
+	desc := fmt.Sprintf("%s's %s calendar", b.dbase.DisplayName(owner), realName)
 	return caldav.Calendar{
 		Path: calHomePath() + localName + "/",
 		// The displayed name is always the plain calendar name (never the
@@ -408,9 +408,10 @@ func (b *Backend) sharedDisplayName(requester, owner, realName string) string {
 	if err != nil {
 		b.logger.Warn("listing own calendars for disambiguation", "error", err)
 	}
+	ownerLabel := b.dbase.DisplayName(owner)
 	for _, c := range own {
 		if c.Name == realName {
-			return fmt.Sprintf("%s (%s)", realName, owner)
+			return fmt.Sprintf("%s (%s)", realName, ownerLabel)
 		}
 	}
 
@@ -423,7 +424,7 @@ func (b *Backend) sharedDisplayName(requester, owner, realName string) string {
 			continue // the calendar itself, not a collision
 		}
 		if sh.CalendarName == realName {
-			return fmt.Sprintf("%s (%s)", realName, owner)
+			return fmt.Sprintf("%s (%s)", realName, ownerLabel)
 		}
 	}
 	return realName