Extend IMAP_PROXY_URL to accept http:// and https:// schemes in addition to the existing socks5:// support. HTTP CONNECT tunneling is used for both new schemes, with TLS to the proxy for https://. Proxy auth via user:pass@ in the URL is supported for all schemes. Refactors dialTLSViaSOCKS5 into a generic dialTLSViaProxy dispatcher. Fixes #9 Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>