fix: check IMAP select() return status before issuing commands

- Add _select_mailbox helper that raises on non-OK status
- Prevents 'command SEARCH illegal in state AUTH' by catching
  failed mailbox selection early with a clear error message

src/email_client.py

@@ -48,10 +48,16 @@ def _get_text_from_msg(msg) -> str:
     return ""
 
 
+def _select_mailbox(conn, mailbox: str = "INBOX"):
+    status, data = conn.select(mailbox)
+    if status != "OK":
+        raise RuntimeError(f"无法选择邮箱 {mailbox}: {data}")
+
+
 def fetch_unseen_emails(account: EmailAccount) -> list[Email]:
     conn = imaplib.IMAP4_SSL(account.imap_server, account.imap_port)
     conn.login(account.username, account.password)
-    conn.select("INBOX")
+    _select_mailbox(conn)
 
     _, data = conn.uid("SEARCH", None, "UNSEEN")
     uids = data[0].split() if data[0] else []
@@ -80,7 +86,7 @@ def mark_as_seen(account: EmailAccount, uids: list[bytes]):
         return
     conn = imaplib.IMAP4_SSL(account.imap_server, account.imap_port)
     conn.login(account.username, account.password)
-    conn.select("INBOX")
+    _select_mailbox(conn)
     for uid in uids:
         conn.uid("STORE", uid, "+FLAGS", "\\Seen")
     conn.logout()