Add sub-account login and ZTE keepalive support

[Tulipyun]

Summary

This PR adds support for CMCC Cloud Computer sub-account users while keeping the existing SMS/password login flow intact.

Changes included:

• Add sub-account password login through the SOHO home sub-account endpoint.
• Persist the selected login mode and sub-account metadata in the existing config flow.
• Support the ZTE firm-auth path returned for sub-account cloud PCs, where getFirmAuth provides VMC/CAG connection fields instead of a normal SCG auth code.
• Implement a pure Go ZTE CAG connection path with TCP/TLS first and UDP/KCP fallback.
• Start the ZTE desktop session, open the CAG proxy link, complete the raw SPICE main-channel handshake, and keep the session alive with SOHO heartbeat plus SPICE auto-replies.
• Handle the raw SPICE server keepalive message observed on the official client path by replying with the matching client-side response.

Verification

• go test ./...
• Manual sub-account login test.
• Manual cloud PC discovery and getFirmAuth test.
• Manual ZTE desktop start and raw SPICE keepalive test for a 150-second session.

[zaq7434]

2026-07-04 00:20:46] [ERROR] ZTE keepalive failed: connectStr missing -h host 测试发现会报这个错了。

[Tulipyun]

你贴个完整的log啊，我测试正常的，你是用的子账号吗？如果不是子账号模式的话我就不知道了，我只有子账号模式的