fix(mail): filter unread messages in beads mode

ListUnread() was returning all messages in beads mode instead of
filtering by the Read field. Apply the same filtering logic used
in legacy mode to both code paths.

Fixes #595

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/mail/mailbox.go

@@ -250,21 +250,17 @@ func (m *Mailbox) listLegacy() ([]*Message, error) {
 
 // ListUnread returns unread (open) messages.
 func (m *Mailbox) ListUnread() ([]*Message, error) {
-	if m.legacy {
-		all, err := m.List()
-		if err != nil {
-			return nil, err
-		}
-		var unread []*Message
-		for _, msg := range all {
-			if !msg.Read {
-				unread = append(unread, msg)
-			}
-		}
-		return unread, nil
+	all, err := m.List()
+	if err != nil {
+		return nil, err
 	}
-	// For beads, inbox only returns open (unread) messages
-	return m.List()
+	var unread []*Message
+	for _, msg := range all {
+		if !msg.Read {
+			unread = append(unread, msg)
+		}
+	}
+	return unread, nil
 }
 
 // Get returns a message by ID.