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/db/users.go

@@ -90,6 +90,19 @@ func (d *DB) SetProfile(username, displayName, email string) error {
 	return requireRowsAffected(res, ErrUserNotFound)
 }
 
+// DisplayName returns username's display name if one is set, otherwise
+// username itself. This is the friendly label to show wherever a user's
+// identity is surfaced in the UI (e.g. "shared by <name>"), instead of
+// the raw login username.
+func (d *DB) DisplayName(username string) string {
+	u, err := d.GetUser(username)
+	if err != nil || u.DisplayName == "" {
+		return username
+	}
+	return u.DisplayName
+}
+
+
 
 // DeleteUser removes username along with all of its calendars, address
 // books, and sharing grants (calendars/addressbooks cascade via foreign