Add outbound websocket client mode (--connect)

[semistrict]

Summary

• Adds --connect <ws://url> option that puts ttyd in client mode: instead of listening for incoming browser connections, ttyd connects outbound to a remote WebSocket server and bridges the local PTY to it
• The remote endpoint receives terminal output (OUTPUT) and can send input (INPUT, RESIZE_TERMINAL, PAUSE, RESUME) using the same binary protocol
• Supports both ws:// and wss:// (TLS) URLs

Usage

# Connect to a remote WebSocket server, sharing a bash session
ttyd --connect ws://relay.example.com:8080/ws -W bash

# With TLS
ttyd --connect wss://relay.example.com/ws -W bash

Use cases

• NAT traversal: ttyd connects outbound to a relay server when it can't listen on a public port
• Service integration: bridge a local terminal to a remote service that accepts WebSocket connections

Changes

• src/server.h: Added connect_url field to server struct
• src/server.c: Added --connect CLI option, client protocol array, and client-mode branching in main() (skips vhost creation, uses lws_client_connect_via_info() instead)
• src/protocol.c: Extracted shared helpers (handle_writeable, receive_append, handle_command, handle_close) used by both server and client callbacks. Added callback_tty_client() for client-mode connections
• test/test-outbound.js: End-to-end test — starts a WS server, connects ttyd in client mode, verifies bidirectional OUTPUT/INPUT and clean disconnect

Test plan

• ttyd --help shows the new --connect option
• Server mode (no --connect) is unchanged
• Client mode against a compatible WebSocket server (OUTPUT, SET_WINDOW_TITLE, SET_PREFERENCES sent)
• Remote input received and executed by local PTY
• Connection error handling (unreachable host, refused connection)
• Clean disconnect (code 1000) on process exit
• wss:// with TLS-enabled endpoint