feat(frontend): add user attributes management UI

Add Vue components and API client for managing user custom attributes.

- Add userAttributes API client with CRUD operations
- Add UserAttributeForm component for displaying/editing attribute values
- Add UserAttributesConfigModal for attribute definition management
- Support all attribute types: text, textarea, number, email, url,
  date, select, multi_select

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/api/admin/index.ts

@@ -15,6 +15,7 @@ import subscriptionsAPI from './subscriptions'
 import usageAPI from './usage'
 import geminiAPI from './gemini'
 import antigravityAPI from './antigravity'
+import userAttributesAPI from './userAttributes'
 
 /**
  * Unified admin API object for convenient access
@@ -31,7 +32,8 @@ export const adminAPI = {
   subscriptions: subscriptionsAPI,
   usage: usageAPI,
   gemini: geminiAPI,
-  antigravity: antigravityAPI
+  antigravity: antigravityAPI,
+  userAttributes: userAttributesAPI
 }
 
 export {
@@ -46,7 +48,8 @@ export {
   subscriptionsAPI,
   usageAPI,
   geminiAPI,
-  antigravityAPI
+  antigravityAPI,
+  userAttributesAPI
 }
 
 export default adminAPI