No way to enter passphrase for encrypted SSH private keys

### Describe the Bug

It appears that Pangolin does not support SSH private keys protected with a passphrase.

When a passphrase-protected private key is configured, authentication fails because there is no way to provide the key's passphrase.

Newt logs show:

```text
DEBUG: nativessh: authenticating user "" (hasPassword=false, hasPrivateKey=true)
DEBUG: nativessh: failed to parse private key for "": ssh: this private key is passphrase protected
DEBUG: nativessh: no password provided for ""
DEBUG: SSH error: Authentication failed
DEBUG: SSH native session error: auth for user "": authentication failed for user ""
```

At the moment, the only apparent workaround is to use an unencrypted private key, which is generally not recommended.

### Environment

- OS Type & Version: Ubuntu 24.04.4 LTS
- Pangolin Version: 1.19.2
- Edition (Community or Enterprise): Enterprise (self-hosted)
- Gerbil Version: 1.4.2
- Traefik Version: 3.7.5
- Newt Version: 1.13.0

### Steps to reproduce
1. Configure SSH access using "Pangolin SSH + Manual Authentication".
2. Configure the target host to use SSH public key authentication.
3. Use a private key protected with a passphrase.
4. Attempt to authenticate using the encrypted private key.

#### Actual Behavior
Authentication fails. There is no prompt or UI element for entering the private key passphrase.

#### Expected Behavior

Pangolin should provide a way to enter the private key passphrase and complete authentication.