Replaced the SSH library + IO thread + intermediate PTY bridge with
vte_terminal_spawn_sync(). This eliminates the broken terminal behavior
(backspace, Ctrl+C, copy/paste, bash completion) caused by the
intermediate PTY with no process on the slave side.
- SshTerminal now spawns 'ssh' as a VTE subprocess
- Removed libssh dependency from terminal (still used for key validation)
- Removed IO thread, manual PTY creation, and thread synchronization
- VTE now handles all terminal I/O, line discipline, resize, etc.
- Password saved: uses sshpass -e with SSHPASS env var if available
- Password not saved: ssh prompts interactively in the terminal
- Removed show_password_dialog (no longer needed)
- Simplified connect flow in ssh_client.cpp