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

src/key_store.cpp

@@ -4,7 +4,6 @@
 #include <cstdlib>
 #include <cstdio>
 #include <cstring>
-#include <libssh/libssh.h>
 
 static std::string trim(const std::string& s) {
 	size_t a = s.find_first_not_of(" \t\r\n");
@@ -128,15 +127,14 @@ bool KeyStore::generate_keypair(const std::string& name, int bits) {
 }
 
 bool KeyStore::import_key(const std::string& name, const std::string& privkey_path) {
-	ssh_key key = nullptr;
-	if (ssh_pki_import_privkey_file(privkey_path.c_str(), nullptr, nullptr, nullptr, &key) != SSH_OK) {
-		return false;
-	}
-	ssh_key_free(key);
+	/* Validate key by running ssh-keygen -l */
+	std::string check_cmd = "ssh-keygen -l -f \"" + privkey_path + "\" >/dev/null 2>&1";
+	int rc = system(check_cmd.c_str());
+	if (rc != 0) return false;
 
 	std::string dest = keys_dir_ + "/" + name;
 	std::string cp_cmd = "cp \"" + privkey_path + "\" \"" + dest + "\" && chmod 600 \"" + dest + "\"";
-	int rc = system(cp_cmd.c_str());
+	rc = system(cp_cmd.c_str());
 	if (rc != 0) return false;
 
 	std::string pub_src = privkey_path + ".pub";