feat: rewrite terminal to use vte_terminal_spawn_sync

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

Makefile

@@ -267,8 +267,6 @@ INSTALL_STRIP_PROGRAM = $(install_sh) -c -s
 LDFLAGS = 
 LIBOBJS = 
 LIBS = 
-LIBSSH_CFLAGS = 
-LIBSSH_LIBS = -lssh
 LTLIBOBJS = 
 MAKEINFO = ${SHELL} '/home/administrator/Programms/ssh/x3-ssh/build-aux/missing' makeinfo
 MKDIR_P = /usr/bin/mkdir -p