Sign in through the server's single sign-on, with a code

The sign-in screen asks the server how it can be signed in to
(GET /api/auth/config) and offers what it finds: the password form, and
"Sign in with <provider>" when the server can do a device login. The TUI
shows a link and a short code, the person approves it in any browser, and
the next poll hands over the ordinary session, so it works over SSH where
no browser can be opened. The terminal never talks to the identity
provider.

The password form is hidden when the server has turned password login
off. `auth: sso` in config.yaml starts the SSO login straight away, but not
right after signing out, where that would sign the person straight back
in; any other value is refused when the config is read. Polling honours the
server's interval, backs off on slow_down, and gives up after repeated
failures rather than retrying forever.

A server without /api/auth/config answers 404 and is treated as passwords
only, so the sign-in screen is the one it had. Needs terdut-server v0.29.0
for SSO.

internal/api/types.go

@@ -32,6 +32,35 @@ type Alert struct {
 	ResolutionSource *string `json:"resolution_source,omitempty"`
 }
 
+// AuthConfig is how the server can be signed in to, from the unauthenticated
+// GET /api/auth/config. A server too old to have the endpoint answers 404, which
+// callers treat as "passwords only".
+type AuthConfig struct {
+	PasswordLogin bool `json:"password_login"`
+	OIDC          struct {
+		Enabled bool   `json:"enabled"`
+		Name    string `json:"name"`
+	} `json:"oidc"`
+
+	// DeviceLogin is whether the server can sign in a client that has no browser,
+	// by showing a code (see StartDeviceLogin).
+	DeviceLogin bool `json:"device_login"`
+}
+
+// DeviceLogin is a sign-in the server has started for this client: the person
+// opens VerificationURL, checks UserCode, and approves; the client polls with
+// DeviceCode until the server hands over a session.
+type DeviceLogin struct {
+	DeviceCode      string `json:"device_code"`
+	UserCode        string `json:"user_code"`
+	VerificationURL string `json:"verification_url"`
+
+	// Interval is how many seconds to wait between polls, and ExpiresIn how many
+	// the person has to approve.
+	Interval  int `json:"interval"`
+	ExpiresIn int `json:"expires_in"`
+}
+
 // Incident statuses.
 const (
 	StatusTriggered    = "triggered"