Cannot connect to Dropbear server due to missing chacha20-poly1305@openssh.com cipher support in cipher list

## Issue Desc
I’m using WindTerm v2.7 on macOS to connect to an OpenWrt device running Dropbear SSH server. The Dropbear configuration prioritizes `chacha20-poly1305@openssh.com` as the encryption cipher. When connecting, WindTerm fails with the following error:
kex error : no match for method encryption server->client: server [chacha20-poly1305@openssh.com], client [aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr,aes256-cbc,aes192-cbc,aes128-cbc,blowfish-cbc,3des-cbc]

WindTerm disconnects immediately after this error.

## Steps to reproduce

1. Set up Dropbear on OpenWrt with: `option extraargs '-c chacha20-poly1305@openssh.com'`
2. Use WindTerm v2.7 on macOS
3. Attempt to connect via SSH using key authentication
4. Observe cipher mismatch and disconnection
WindTerm should either:
- Support `chacha20-poly1305@openssh.com` in its cipher list on macOS, or
- Allow users to manually enter unsupported ciphers in the session config

## Additional context

The same connection works fine from OpenSSH terminal (Zsh) on macOS.

Start by tracing the SSH cipher negotiation and macOS cipher-list configuration implicated by the error. Reproduce against Dropbear configured with chacha20-poly1305@openssh.com and compare WindTerm’s offered algorithms with OpenSSH. Done means WindTerm connects with that cipher or exposes a session setting for it.

Written by the indexing model from the issue text.