Add invalid-request fallback routing

frontend/src/i18n/locales/en.ts

@@ -919,6 +919,11 @@ export default {
         fallbackHint: 'Non-Claude Code requests will use this group. Leave empty to reject directly.',
         noFallback: 'No Fallback (Reject)'
       },
+      invalidRequestFallback: {
+        title: 'Invalid Request Fallback Group',
+        hint: 'Triggered only when upstream explicitly returns prompt too long. Leave empty to disable fallback.',
+        noFallback: 'No Fallback'
+      },
       modelRouting: {
         title: 'Model Routing',
         tooltip: 'Configure specific model requests to be routed to designated accounts. Supports wildcard matching, e.g., claude-opus-* matches all opus models.',