fix: strategy studio black screen on create and remove stale benefit3 ref

- Add missing configResponse.ok check in handleCreateStrategy to prevent
  rendering with invalid config data when API fails
- Remove deleted benefit3 translation key reference from LoginRequiredOverlay

web/src/components/auth/LoginRequiredOverlay.tsx

@@ -23,7 +23,6 @@ export function LoginRequiredOverlay({ isOpen, onClose, featureName }: LoginRequ
   const benefits = [
     tr('benefit1'),
     tr('benefit2'),
-    tr('benefit3'),
     tr('benefit4'),
   ]
 

web/src/pages/StrategyStudioPage.tsx

@@ -201,6 +201,7 @@ export function StrategyStudioPage() {
         `${API_BASE}/api/strategies/default-config?lang=${language}`,
         { headers: { Authorization: `Bearer ${token}` } }
       )
+      if (!configResponse.ok) throw new Error('Failed to fetch default config')
       const defaultConfig = await configResponse.json()
 
       const response = await fetch(`${API_BASE}/api/strategies`, {