feat(usage): add User-Agent column to usage logs

- Add user_agent field to UsageLog DTO and mapper
- Display User-Agent column in admin and user usage tables
- Add formatUserAgent helper to show friendly client names
- Include user_agent in Excel export
- Remove request_id column from admin usage table

frontend/src/i18n/locales/en.ts

@@ -424,7 +424,8 @@ export default {
     billingType: 'Billing',
     balance: 'Balance',
     subscription: 'Subscription',
-    imageUnit: ' images'
+    imageUnit: ' images',
+    userAgent: 'User-Agent'
   },
 
   // Redeem

frontend/src/i18n/locales/zh.ts

@@ -421,7 +421,8 @@ export default {
     billingType: '消费类型',
     balance: '余额',
     subscription: '订阅',
-    imageUnit: '张'
+    imageUnit: '张',
+    userAgent: 'User-Agent'
   },
 
   // Redeem