Add visually-hidden h1 headings to login and signup pages

frontend/editor/src/proprietary/routes/Login.tsx

@@ -564,6 +564,7 @@ export default function Login() {
   return (
     <AuthLayout>
       <div className="auth-logo-block">
+        <h1 className="auth-visually-hidden">{t("login.title", "Sign in")}</h1>
         <img
           src={withBasePath("/modern-logo/LoginLightModeHeader.svg")}
           alt="Stirling PDF"

frontend/editor/src/proprietary/routes/Signup.tsx

@@ -92,6 +92,7 @@ export default function Signup() {
   return (
     <AuthLayout>
       <div className="auth-logo-block">
+        <h1 className="auth-visually-hidden">{t("signup.title", "Create an account")}</h1>
         <img
           src={withBasePath("/modern-logo/LoginLightModeHeader.svg")}
           alt="Stirling PDF"

frontend/editor/src/proprietary/routes/authShared/auth.css

@@ -635,6 +635,18 @@
   display: none;
 }
 
+.auth-visually-hidden {
+  position: absolute;
+  width: 1px;
+  height: 1px;
+  padding: 0;
+  margin: -1px;
+  overflow: hidden;
+  clip: rect(0, 0, 0, 0);
+  white-space: nowrap;
+  border: 0;
+}
+
 /* ── Page entrance animation ────────────────────────────────────────── */
 @keyframes authFadeUp {
   from {