Add SOCKS5/HTTP proxy support for SSH connections

Allow SSH connections to be routed through SOCKS5 or HTTP CONNECT proxies.

Backend:
- Add ProxyType, ProxyHost, ProxyPort, ProxyUsername, ProxyPassword to SSHConfig
- dialTarget() dispatches to direct TCP, SOCKS5, or HTTP CONNECT based on config
- SOCKS5: uses golang.org/x/net/proxy with optional username/password auth
- HTTP CONNECT: sends CONNECT request with optional Basic auth, validates 200 OK
- Proxy dialling replaces ssh.Dial for non-jump-host connections; the resulting
  net.Conn is passed to ssh.NewClientConn for the SSH handshake

Frontend:
- Add Proxy section to SSH profile editor (collapsed by default)
- Proxy type selector: None / SOCKS5 / HTTP CONNECT
- Host, port, username, password fields
- Collect proxy values on save

internal/models/models.go

@@ -99,6 +99,11 @@ type SSHConfig struct {
 	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)
+	ProxyType         string            `json:"proxyType"`        // "none", "socks5", "http"
+	ProxyHost         string            `json:"proxyHost"`
+	ProxyPort         int               `json:"proxyPort"`
+	ProxyUsername     string            `json:"proxyUsername"`
+	ProxyPassword     string            `json:"proxyPassword"` // stored in vault for production; inline for simplicity
 	JumpHosts         []JumpHost        `json:"jumpHosts"`
 	PortForwards      []PortForward     `json:"portForwards"`
 	StartupCommand    string            `json:"startupCommand"`