feat: Implement directory validation and shell command quoting

- Added is_valid_directory function to validate directory paths, ensuring they do not contain control characters and are within a specified length.
- Introduced quote_shell_arg to safely quote shell arguments, preventing command injection.
- Created build_cd_command to generate a command for changing directories in a shell.
- Enhanced the LoginHandler to utilize a login rate limiter, preventing brute-force attacks by tracking failed login attempts.
- Implemented an EncodingCache to optimize encoding detection for SSH connections.
- Updated the UI to include an input field for specifying an initial directory upon login, with appropriate validation and hints.
- Added a quickbar in the terminal interface for easy access to copy and paste functionality.
- Introduced a toast notification system to provide feedback on copy actions.
- Refactored connection storage to encrypt passwords at rest, improving security.
- Updated various templates and styles to accommodate new features and improve user experience.

tests/sshserver.py

@@ -20,6 +20,9 @@ print('Read key: ' + hexlify(host_key.get_fingerprint()).decode('utf-8'))
 
 banner = u'\r\n\u6b22\u8fce\r\n'
 event_timeout = 5
+# clients that reuse a cached encoding never send the probe command, so this
+# wait must be short and non-fatal
+exec_timeout = 1
 
 
 class Server(paramiko.ServerInterface):
@@ -41,6 +44,7 @@ class Server(paramiko.ServerInterface):
         self.shell_event = threading.Event()
         self.exec_event = threading.Event()
         self.cmd_to_enc = self.get_cmd2enc(encodings)
+        self.encoding = 'UTF-8'
         self.password_verified = False
         self.key_verified = False
 
@@ -171,10 +175,11 @@ def run_ssh_server(port=2200, running=True, encodings=[]):
             print('*** Client never asked for a shell.')
             continue
 
-        server.exec_event.wait(timeout=event_timeout)
+        server.exec_event.wait(timeout=exec_timeout)
         if not server.exec_event.is_set():
+            # the client already knew this server's encoding and skipped the
+            # probe; fall back to the default instead of dropping the session
             print('*** Client never asked for a command.')
-            continue
 
         # chan.send('\r\n\r\nWelcome!\r\n\r\n')
         print(server.encoding)