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.',

frontend/src/i18n/locales/zh.ts

@@ -995,6 +995,11 @@ export default {
         fallbackHint: '非 Claude Code 请求将使用此分组，留空则直接拒绝',
         noFallback: '不降级（直接拒绝）'
       },
+      invalidRequestFallback: {
+        title: '无效请求兜底分组',
+        hint: '仅当上游明确返回 prompt too long 时才会触发，留空表示不兜底',
+        noFallback: '不兜底'
+      },
       modelRouting: {
         title: '模型路由配置',
         tooltip: '配置特定模型请求优先路由到指定账号。支持通配符匹配，如 claude-opus-* 匹配所有 opus 模型。',