fix: normalize null fields from unconfigured OIDC response

When no OIDC config exists, the backend returns an object with all
null fields (via OidcAdminConfigResponse.unconfigured()). Normalize
all null values to sensible defaults when loading the form instead
of passing nulls through to Input components and .map() calls.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/Admin/OidcConfigPage.tsx

@@ -38,8 +38,17 @@ export default function OidcConfigPage() {
   const { toast } = useToast();
 
   useEffect(() => {
-    adminFetch<OidcFormData>('/oidc')
-      .then(setForm)
+    adminFetch<Partial<OidcFormData> & { configured?: boolean }>('/oidc')
+      .then((data) => setForm({
+        enabled: data.enabled ?? false,
+        autoSignup: data.autoSignup ?? true,
+        issuerUri: data.issuerUri ?? '',
+        clientId: data.clientId ?? '',
+        clientSecret: data.clientSecret ?? '',
+        rolesClaim: data.rolesClaim ?? 'roles',
+        displayNameClaim: data.displayNameClaim ?? 'name',
+        defaultRoles: data.defaultRoles ?? ['VIEWER'],
+      }))
       .catch(() => setForm(EMPTY_CONFIG));
   }, []);