Add TOTP/2FA support for SSH keyboard-interactive auth

Automatically generate and submit TOTP codes during SSH keyboard-interactive
authentication when a secret is configured for the profile.

Backend:
- Add TOTPSecret field to SSHConfig model
- Detect TOTP/2FA prompts in keyboard-interactive handler using keyword
  matching (verification code, otp, authenticator, 2fa, etc.)
- Generate 6-digit TOTP codes using pquerna/otp library
- Retrieve TOTP secret from vault during connection (keychain or file vault)
- API: SetTOTPSecret, GetTOTPCode, HasTOTPSecret, DeleteTOTPSecret

Frontend:
- Add TOTP/2FA section to SSH profile editor (collapsed by default)
- Password-masked secret input with show/hide toggle
- Save button: encrypts and stores in vault
- Test button: generates a live 6-digit code (shown for 5 seconds)
- Delete button: removes stored secret
- Auto-detect whether secret is already saved (placeholder indicator)
- CSS for TOTP input row, danger button style

internal/models/models.go

@@ -98,6 +98,7 @@ type SSHConfig struct {
 	AutoReconnect     bool              `json:"autoReconnect"`     // reconnect on unexpected disconnect
 	MaxReconnectTries int               `json:"maxReconnectTries"` // 0 = use default (5)
 	LoginScripts      []LoginScriptStep `json:"loginScripts"`     // expect-style auto-login sequence
+	TOTPSecret        string            `json:"totpSecret"`       // TOTP secret key for 2FA auto-fill (stored encrypted)
 	JumpHosts         []JumpHost        `json:"jumpHosts"`
 	PortForwards      []PortForward     `json:"portForwards"`
 	StartupCommand    string            `json:"startupCommand"`