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/config/config_test.go

@@ -59,3 +59,26 @@ func TestLoad_NoAPIKeyNeeded(t *testing.T) {
 		t.Error("expected the leftover api_key to be noted")
 	}
 }
+
+func TestLoad_AuthIsOptionalAndChecked(t *testing.T) {
+	for _, tc := range []struct {
+		yaml, want string
+		bad        bool
+	}{
+		{"", "", false},
+		{"auth: password\n", "password", false},
+		{"auth: sso\n", "sso", false},
+		{"auth: oidc\n", "", true},
+	} {
+		writeConfig(t, "server_url: https://terdut.example.com\n"+tc.yaml)
+		cfg, err := Load()
+		switch {
+		case tc.bad && err == nil:
+			t.Errorf("%q: expected an error", tc.yaml)
+		case !tc.bad && err != nil:
+			t.Errorf("%q: %v", tc.yaml, err)
+		case !tc.bad && cfg.Auth != tc.want:
+			t.Errorf("%q: auth %q, want %q", tc.yaml, cfg.Auth, tc.want)
+		}
+	}
+}