feat(groups): add Claude Code client restriction and session isolation

- Add claude_code_only field to restrict groups to Claude Code clients only
- Add fallback_group_id for non-Claude Code requests to use alternate group
- Implement ClaudeCodeValidator for User-Agent detection
- Add group-level session binding isolation (groupID in Redis key)
- Prevent cross-group sticky session pollution
- Update frontend with Claude Code restriction controls

frontend/src/i18n/locales/en.ts

@@ -857,6 +857,15 @@ export default {
       imagePricing: {
         title: 'Image Generation Pricing',
         description: 'Configure pricing for gemini-3-pro-image model. Leave empty to use default prices.'
+      },
+      claudeCode: {
+        title: 'Claude Code Client Restriction',
+        tooltip: 'When enabled, this group only allows official Claude Code clients. Non-Claude Code requests will be rejected or fallback to the specified group.',
+        enabled: 'Claude Code Only',
+        disabled: 'Allow All Clients',
+        fallbackGroup: 'Fallback Group',
+        fallbackHint: 'Non-Claude Code requests will use this group. Leave empty to reject directly.',
+        noFallback: 'No Fallback (Reject)'
       }
     },