Detect backend down (#5010)

# Description of Changes

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Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
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---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

frontend/src/proprietary/routes/Login.tsx

@@ -1,11 +1,13 @@
 import { useEffect, useState } from 'react';
-import { useNavigate, useSearchParams } from 'react-router-dom';
+import { Navigate, useNavigate, useSearchParams } from 'react-router-dom';
 import { Text, Stack, Alert } from '@mantine/core';
 import { springAuth } from '@app/auth/springAuthClient';
 import { useAuth } from '@app/auth/UseSession';
+import { useAppConfig } from '@app/contexts/AppConfigContext';
 import { useTranslation } from 'react-i18next';
 import { useDocumentMeta } from '@app/hooks/useDocumentMeta';
 import AuthLayout from '@app/routes/authShared/AuthLayout';
+import { useBackendProbe } from '@app/hooks/useBackendProbe';
 
 // Import login components
 import LoginHeader from '@app/routes/login/LoginHeader';
@@ -20,18 +22,41 @@ export default function Login() {
   const navigate = useNavigate();
   const [searchParams] = useSearchParams();
   const { session, loading } = useAuth();
+  const { refetch } = useAppConfig();
   const { t } = useTranslation();
   const [isSigningIn, setIsSigningIn] = useState(false);
   const [error, setError] = useState<string | null>(null);
   const [successMessage, setSuccessMessage] = useState<string | null>(null);
   const [showEmailForm, setShowEmailForm] = useState(false);
-  const [email, setEmail] = useState('');
+  const [email, setEmail] = useState(() => searchParams.get('email') ?? '');
   const [password, setPassword] = useState('');
   const [enabledProviders, setEnabledProviders] = useState<string[]>([]);
   const [hasSSOProviders, setHasSSOProviders] = useState(false);
   const [_enableLogin, setEnableLogin] = useState<boolean | null>(null);
+  const backendProbe = useBackendProbe();
   const [isFirstTimeSetup, setIsFirstTimeSetup] = useState(false);
   const [showDefaultCredentials, setShowDefaultCredentials] = useState(false);
+  const loginDisabled = backendProbe.loginDisabled === true || _enableLogin === false;
+
+  // Periodically probe while backend isn't up so the screen can auto-advance when it comes online
+  useEffect(() => {
+    if (backendProbe.status === 'up' || backendProbe.loginDisabled) {
+      return;
+    }
+    const tick = async () => {
+      const result = await backendProbe.probe();
+      if (result.status === 'up') {
+        await refetch();
+        if (loginDisabled) {
+          navigate('/', { replace: true });
+        }
+      }
+    };
+    const intervalId = window.setInterval(() => {
+      void tick();
+    }, 5000);
+    return () => window.clearInterval(intervalId);
+  }, [backendProbe.status, backendProbe.loginDisabled, backendProbe.probe, refetch, navigate, loginDisabled]);
 
   // Redirect immediately if user has valid session (JWT already validated by AuthProvider)
   useEffect(() => {
@@ -41,6 +66,21 @@ export default function Login() {
     }
   }, [session, loading, navigate]);
 
+  // If backend reports login is disabled, redirect to home (anonymous mode)
+  useEffect(() => {
+    if (backendProbe.loginDisabled) {
+      // Slight delay to allow state updates before redirecting
+      const id = setTimeout(() => navigate('/', { replace: true }), 0);
+      return () => clearTimeout(id);
+    }
+  }, [backendProbe.loginDisabled, navigate]);
+
+  useEffect(() => {
+    if (backendProbe.status === 'up') {
+      void refetch();
+    }
+  }, [backendProbe.status, refetch]);
+
   // Fetch enabled SSO providers and login config from backend
   useEffect(() => {
     const fetchProviders = async () => {
@@ -73,8 +113,11 @@ export default function Login() {
         console.error('[Login] Failed to fetch enabled providers:', err);
       }
     };
-    fetchProviders();
-  }, [navigate]);
+
+    if (backendProbe.status === 'up' || backendProbe.loginDisabled) {
+      fetchProviders();
+    }
+  }, [navigate, backendProbe.status, backendProbe.loginDisabled]);
 
   // Update hasSSOProviders and showEmailForm when enabledProviders changes
   useEffect(() => {
@@ -134,11 +177,55 @@ export default function Login() {
     ogUrl: `${window.location.origin}${window.location.pathname}`
   });
 
+  // If login is disabled, short-circuit to home (avoids rendering the form after retry)
+  if (loginDisabled) {
+    return <Navigate to="/" replace />;
+  }
+
   // Show logged in state if authenticated
   if (session && !loading) {
     return <LoggedInState />;
   }
 
+  // If backend isn't ready yet, show a lightweight status screen instead of the form
+  if (backendProbe.status !== 'up' && !loginDisabled) {
+    const backendTitle = t('backendStartup.notFoundTitle', 'Backend not found');
+    const handleRetry = async () => {
+      const result = await backendProbe.probe();
+      if (result.status === 'up') {
+        await refetch();
+        navigate('/', { replace: true });
+      }
+    };
+    return (
+      <AuthLayout>
+        <LoginHeader title={backendTitle} />
+        <div
+          className="auth-section"
+          style={{
+            padding: '1.5rem',
+            marginTop: '1rem',
+            borderRadius: '0.75rem',
+            backgroundColor: 'rgba(37, 99, 235, 0.08)',
+            border: '1px solid rgba(37, 99, 235, 0.2)',
+          }}
+        >
+          <p style={{ margin: '0 0 0.75rem 0', color: 'rgba(15, 23, 42, 0.8)' }}>
+            {t('backendStartup.unreachable')}
+          </p>
+          <button
+            type="button"
+            onClick={handleRetry}
+            className="auth-cta-button px-4 py-[0.75rem] rounded-[0.625rem] text-base font-semibold mt-5 border-0 cursor-pointer"
+            style={{ width: 'fit-content' }}
+          >
+            {t('backendStartup.retry', 'Retry')}
+          </button>
+        </div>
+      </AuthLayout>
+    );
+  }
+
   const signInWithProvider = async (provider: 'github' | 'google' | 'apple' | 'azure' | 'keycloak' | 'oidc') => {
     try {
       setIsSigningIn(true);