feat(#138): support proxy binding in /auth/login

POST /auth/login now accepts optional proxy field for both single
and batch account creation. Proxy is parsed and bound to the newly
created account as per-account proxy.

Single:  {"token":"...", "proxy":"socks5://1.2.3.4:1080"}
Batch:   {"accounts":[{"token":"t1","proxy":"http://p:8080"},...]}

Verified on VPS — account created with proxy bound in proxy.json.