fix: mark_as_seen uses readonly=False to actually set Seen flag

src/email_client.py

@@ -204,7 +204,7 @@ def mark_as_seen(account: EmailAccount, uids_with_folder: list[tuple[bytes, str]
         by_folder[folder].append(uid)
     for folder, uids in by_folder.items():
         try:
-            conn.select(folder, readonly=True)
+            conn.select(folder, readonly=False)
             for uid in uids:
                 conn.uid("STORE", uid, "+FLAGS", "\\Seen")
             logger.info("  %s: 标记 %d 封", folder, len(uids))