fix: add IMAP ID command for 126.com/163.com support

These providers require a client identification (ID command, RFC 2971)
before SELECT INBOX, otherwise they reject with 'SELECT Unsafe Login'.
Extract shared login+select logic into _login_and_prepare helper.

src/email_client.py

@@ -48,16 +48,36 @@ def _get_text_from_msg(msg) -> str:
     return ""
 
 
+_PROVIDERS_NEED_ID = ("163.com", "126.com")
+
+
+def _check_provider(email_addr: str, providers: tuple[str, ...]) -> bool:
+    return any(email_addr.endswith(p) for p in providers)
+
+
+def _send_id_command(conn):
+    imaplib.Commands["ID"] = "AUTH"
+    args = ("name", "AI-Mail-Bot", "version", "1.0.0", "vendor", "personal")
+    conn._simple_command("ID", '("' + '" "'.join(args) + '")')
+
+
 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]:
+def _login_and_prepare(account: EmailAccount):
     conn = imaplib.IMAP4_SSL(account.imap_server, account.imap_port)
     conn.login(account.username, account.password)
+    if _check_provider(account.username, _PROVIDERS_NEED_ID):
+        _send_id_command(conn)
     _select_mailbox(conn)
+    return conn
+
+
+def fetch_unseen_emails(account: EmailAccount) -> list[Email]:
+    conn = _login_and_prepare(account)
 
     _, data = conn.uid("SEARCH", None, "UNSEEN")
     uids = data[0].split() if data[0] else []
@@ -84,9 +104,7 @@ def fetch_unseen_emails(account: EmailAccount) -> list[Email]:
 def mark_as_seen(account: EmailAccount, uids: list[bytes]):
     if not uids:
         return
-    conn = imaplib.IMAP4_SSL(account.imap_server, account.imap_port)
-    conn.login(account.username, account.password)
-    _select_mailbox(conn)
+    conn = _login_and_prepare(account)
     for uid in uids:
         conn.uid("STORE", uid, "+FLAGS", "\\Seen")
     conn.logout()