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

@@ -250,7 +250,6 @@ 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
@@ -262,9 +261,6 @@ func (m *Mailbox) ListUnread() ([]*Message, error) {
 		}
 	}
 	return unread, nil
-	}
-	// For beads, inbox only returns open (unread) messages
-	return m.List()
 }
 
 // Get returns a message by ID.