feat(mail): implement resolveThreadID with References fallback

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/mail/imap.go

@@ -283,6 +283,42 @@ func (w *IMAPWatcher) fetchUnseen(c *imapclient.Client) error {
 	return nil
 }
 
+// resolveThreadID determines the thread_id for a message.
+// It tries In-Reply-To first, then falls back to References (last-to-first).
+// Returns the message's own ID if no parent is found.
+func resolveThreadID(messageID, inReplyTo, references string, lookup func(string) (*database.Task, error)) string {
+	if inReplyTo != "" {
+		if parent, err := lookup(inReplyTo); err == nil && parent != nil {
+			return parent.ThreadID
+		}
+	}
+
+	for _, refID := range parseReferences(references) {
+		if parent, err := lookup(refID); err == nil && parent != nil {
+			return parent.ThreadID
+		}
+	}
+
+	return messageID
+}
+
+// parseReferences splits a References header value into individual message IDs,
+// returned in reverse order (most recent first).
+func parseReferences(header string) []string {
+	if header == "" {
+		return nil
+	}
+	raw := strings.Fields(header)
+	for i := range raw {
+		raw[i] = strings.TrimSpace(raw[i])
+	}
+	// Reverse: last (most recent) first.
+	for i, j := 0, len(raw)-1; i < j; i, j = i+1, j-1 {
+		raw[i], raw[j] = raw[j], raw[i]
+	}
+	return raw
+}
+
 // processMessage handles a single fetched email message.
 func (w *IMAPWatcher) processMessage(c *imapclient.Client, buf *imapclient.FetchMessageBuffer, bodySection *imap.FetchItemBodySection) {
 	env := buf.Envelope