Desktop/remove hard requirement auth wall on desktop (#5956)

Co-authored-by: Claude Sonnet 4.6 <[email protected]>

frontend/src/desktop/routes/Landing.tsx

@@ -0,0 +1,12 @@
+import HomePage from '@app/pages/HomePage';
+
+/**
+ * Desktop override of Landing.
+ * In desktop builds, authentication is managed entirely by AppProviders,
+ * the DesktopOnboardingModal, and the SignInModal — never by routing to /login.
+ * Always render the main app; the onboarding/sign-in modals appear on top
+ * when authentication is required.
+ */
+export default function Landing() {
+  return <HomePage />;
+}

frontend/src/desktop/routes/Login.tsx

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+import { Navigate } from 'react-router-dom';
+
+/**
+ * Desktop override of the /login route.
+ * The legacy web login page must never appear in desktop builds — authentication
+ * is handled exclusively through the DesktopOnboardingModal and SignInModal.
+ * Any navigation to /login (e.g. from Spring Boot auth redirects) is intercepted
+ * here and immediately redirected to /.
+ * The sign-in modal is opened by the desktop httpErrorHandler before navigation
+ * occurs, so no additional dispatch is needed here.
+ */
+export default function Login() {
+  return <Navigate to="/" replace />;
+}

frontend/src/desktop/routes/login/LoginHeader.tsx

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+import { ActionIcon } from '@mantine/core';
+import CloseIcon from '@mui/icons-material/Close';
+import { useLogoAssets } from '@app/hooks/useLogoAssets';
+
+interface LoginHeaderProps {
+  title: string;
+  subtitle?: string;
+  centerOnly?: boolean;
+  onClose?: () => void;
+}
+
+/**
+ * Desktop override of LoginHeader.
+ * Renders icon + title + optional close button all in one row.
+ */
+export default function LoginHeader({ title, subtitle, centerOnly = false, onClose }: LoginHeaderProps) {
+  const { tooltipLogo } = useLogoAssets();
+
+  return (
+    <div className={`login-header${centerOnly ? ' login-header-centered' : ''}`} style={{ marginBottom: '2rem' }}>
+      <div style={{ display: 'flex', alignItems: 'center', justifyContent: 'space-between', gap: '0.75rem' }}>
+        <div style={{ display: 'flex', alignItems: 'center', gap: '0.6rem', flex: 1, minWidth: 0 }}>
+          <img src={tooltipLogo} alt="Stirling PDF" style={{ width: 36, height: 36, flexShrink: 0 }} />
+          {title && <h1 className="login-title" style={{ margin: 0 }}>{title}</h1>}
+        </div>
+        {onClose && (
+          <ActionIcon
+            onClick={onClose}
+            radius="md"
+            size={32}
+            variant="subtle"
+            style={{ flexShrink: 0, color: 'var(--text-secondary)', outline: 'none' }}
+          >
+            <CloseIcon fontSize="small" />
+          </ActionIcon>
+        )}
+      </div>
+      {subtitle && <p className="login-subtitle">{subtitle}</p>}
+    </div>
+  );
+}