Feature: Add admin-only SSO bridge to web UI

Bridge an already-authenticated portal session into a YAMA web session via
a new /sso endpoint, so portal administrators can enter the remote-desktop
UI without a second login. Only the "admin" portal role is bridged; any
other role (viewer, unknown) still falls back to the built-in login form,
so a low-privileged portal account cannot bypass authentication.

- wsauth: extract IssueToken from VerifyLogin so an out-of-band-authenticated
  identity (SSO) can mint a session token without the challenge/response.
- web: add the SSODetector interface, a fail-closed JWT detector (HMAC-only),
  and the /sso handler gated to the admin role.
- cmd: enable the JWT detector via YAMA_SSO_JWT_SECRET, the shared HS256
  secret matching the parent portal.
- web UI: attempt SSO before showing the login form, falling back to login
  when the server returns 401.
- docs: document SSO (admin-only) and license trial-vs-paid troubleshooting
  in README and the env example.

Co-Authored-By: deepseek-v4-pro

server/go/web/sso.go

@@ -0,0 +1,63 @@
+package web
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+// SSODetector maps an incoming HTTP request from an already-authenticated
+// portal session into a YAMA web identity (username + role). Implementations
+// are portal-specific: each one knows how to recognize its portal's session
+// credential (a cookie, a header, a JWT, ...) and translate the caller to
+// YAMA's two roles, "admin" and "viewer".
+//
+// Detect returns ok=false when the request carries no usable credential, or
+// the credential is invalid/expired; the caller then falls back to the
+// built-in login form. username is used only for audit/logging. Whether a
+// detected identity is actually bridged into a session is handleSSO's policy:
+// currently only "admin" is bridged, everyone else falls back to the login form.
+type SSODetector interface {
+	Detect(r *http.Request) (username, role string, ok bool)
+}
+
+// handleSSO bridges a detected portal identity into a YAMA session token.
+// Only administrators are bridged: a valid portal session for any other role
+// (viewer, unknown) still returns 401 and the front-end falls back to the
+// built-in login form, so a low-privileged portal account cannot walk straight
+// into the remote-desktop console. When no detector is configured, or the
+// request is not recognized, it also returns 401.
+func (s *Server) handleSSO(w http.ResponseWriter, r *http.Request) {
+	if s.sso == nil {
+		s.unauthorized(w)
+		return
+	}
+	username, role, ok := s.sso.Detect(r)
+	if !ok {
+		s.unauthorized(w)
+		return
+	}
+	// Only the "admin" role may skip the login form via SSO. Every other role
+	// (viewer, unknown) must authenticate against YAMA's own login, even with
+	// a valid portal session.
+	if role != "admin" {
+		s.unauthorized(w)
+		return
+	}
+
+	token, err := s.auth.IssueToken(username, role)
+	if err != nil {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusInternalServerError)
+		_, _ = w.Write([]byte(`{"error":"internal error"}`))
+		return
+	}
+
+	s.log.Info("sso: user=%s role=%s", username, role)
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.Header().Set("Cache-Control", "no-store")
+	_ = json.NewEncoder(w).Encode(map[string]string{
+		"token":    token,
+		"role":     role,
+		"username": username,
+	})
+}