Guard against empty From header in IMAP message fetch

Bounce messages and MAILER-DAEMON notices can have no From address,
causing a panic on the bare [0] index access.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/email/fetch.go

@@ -74,6 +74,9 @@ func fetchEmails(imapClient *client.Client) ([]Email, error) {
 		}()
 
 		for msg := range messages {
+			if len(msg.Envelope.From) == 0 {
+				continue
+			}
 			toAddress := concatAddresses(msg.Envelope.To)
 			fromAddress := msg.Envelope.From[0].Address()
 			r := msg.GetBody(section)