fix(login): show friendly message for upstream OAuth failures (#300)

Display a user-friendly error when redirected back with ?error=upstream
after a transient identity provider failure, instead of showing nothing.

src/app/(main)/login/page.tsx

···
 
   const [handle, setHandle] = useState('');
   const [isSubmitting, setIsSubmitting] = useState(false);
-  const [error, setError] = useState<string | null>(null);
+  const upstreamError = searchParams.get('error') === 'upstream' ? t('errorUpstream') : null;
+  const [error, setError] = useState<string | null>(upstreamError);
 
   useEffect(() => {
     if (session && !isLoading) {

src/i18n/locales/en.json

···
     "submitting": "Signing in...",
     "errorHandleNotFound": "Could not find an account for \"{handle}\". Enter your full handle including the domain (e.g. \"jay.bsky.social\" or \"alice.custom-pds.example\"). A username alone (e.g. \"jay\") only works for Bluesky accounts.",
     "errorGeneric": "Failed to sign in. Please check your handle and try again.",
-    "errorNetwork": "Network error. Please try again."
+    "errorNetwork": "Network error. Please try again.",
+    "errorUpstream": "Sign-in failed due to a temporary issue with the identity provider. Please try again."
   },
   "about": {
     "title": "About Sifa",