Cannot connect to imap.sina.cn — rustls doesn't support DHE cipher suites

[yzgolden86]

报错：
TLS handshake with imap.sina.cn failed: received fatal alert: HandshakeFailure

sina.cn IMAP server only offers DHE-RSA-AES256-GCM-SHA384 for TLS 1.2 and rustls doesn't support DHE. Could you provide a native-tls feature flag as alternative TLS backend?

[QingJ01]

已在 74d9203 修复并合入 master。现在 IMAP 隐式 TLS 和 STARTTLS 都会先尝试 rustls；如果握手失败，会重新连接并 fallback 到 native-tls，以覆盖 imap.sina.cn 这类只提供 DHE cipher suites、rustls 不支持的服务器。连接诊断 test_connection 也会报告 native-tls fallback 结果。\n\n本地核实：cargo test -p pebble-mail 通过，153 passed。