fix(mail): drain UidFetch channel on early error; clarify ParseHeaderOnly doc

If header/body parsing errored mid-fetch we returned without draining the
message channel, so the UidFetch goroutine could block on a full channel.
Both fetch paths now break, drain remaining messages, then read the done
error. Verified with the race detector.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/mail/message.go

@@ -95,7 +95,9 @@ func ParseMessage(uid uint32, raw []byte) (Message, error) {
 	return m, nil
 }
 
-// ParseHeaderOnly decodes headers and detects attachments without keeping bodies.
+// ParseHeaderOnly parses a full RFC822 message (raw bytes) and returns only the
+// Header, with HasAttachments set. For IMAP BODY[HEADER]-only bytes (no body),
+// use ParseHeaderBytes instead, which avoids downloading the body entirely.
 func ParseHeaderOnly(uid uint32, raw []byte) (Header, error) {
 	m, err := ParseMessage(uid, raw)
 	if err != nil {