feat(auth): add native SAML 2.0 SSO integration

Add SAML 2.0 as a second SSO protocol alongside OIDC under a unified
authMode/ssoType model. SP flows via @node-saml/node-saml: AuthnRequest
generation, ACS POST assertion handling, SP metadata export, and admin
config test endpoint. Replay-protected via saml_state cookie (httpOnly,
SameSite=Lax) matched against InResponseTo; wantAssertionsSigned enforced.

- src/lib/auth/saml.js: SAML instance builder, X.509 cert formatter, claim pickers
- 4 routes under src/app/api/auth/saml/: start, acs, metadata, test
- settingsRepo: ssoType + saml* defaults; login/status routes dispatch by type
- profile page: SSO protocol switcher, IdP metadata XML + cert uploaders
- login page: dynamic SAML sign-in button; Header: SAML user badge

src/dashboardGuard.js

@@ -27,6 +27,7 @@ const PUBLIC_API_PATHS = [
   "/api/auth/logout",
   "/api/auth/status",
   "/api/auth/oidc",
+  "/api/auth/saml",
   "/api/version",
   "/api/settings/require-login",
 ];