Switch tungstenite from native-tls to rustls — no OpenSSL dependency

native-tls requires libssl-dev + pkg-config on Linux. Switching to
rustls-tls-native-roots means the project builds on Linux with no
system OpenSSL installation needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: SyntaxNyah