Phase 1: Increase Argon2id to 128 MiB and add SSH agent forwarding

Argon2id:
- Increase memory parameter from 64 MiB to 128 MiB for stronger KDF

SSH Agent:
- Add agent.go with connectAgent(), agentAuthMethod(), requestAgentForwarding()
- Integrate agent auth into buildAuthMethods() (tried first when enabled)
- Set up agent forwarding after PTY allocation when agentForwarding is true
- Track and clean up agent socket connection on session close
- Add AgentAuth and AgentForwarding fields to SSHConfig model
- Add SSH Agent section to profile editor with toggle controls

internal/models/models.go

@@ -93,6 +93,8 @@ type SSHConfig struct {
 	KeepaliveInterval int               `json:"keepaliveInterval"`
 	KeepaliveCountMax int               `json:"keepaliveCountMax"`
 	Compression       bool              `json:"compression"`
+	AgentAuth         bool              `json:"agentAuth"`         // use SSH agent for authentication
+	AgentForwarding   bool              `json:"agentForwarding"`   // forward agent to remote host
 	JumpHosts         []JumpHost        `json:"jumpHosts"`
 	PortForwards      []PortForward     `json:"portForwards"`
 	StartupCommand    string            `json:"startupCommand"`