Merge pull request #3718 from wucm667/fix/usage-csv-export-utf8-bom

fix(usage): CSV 导出补 UTF-8 BOM，修复 Excel 打开中文列乱码

frontend/src/views/user/UsageView.vue

@@ -677,7 +677,7 @@ const exportToCSV = async () => {
       headers.map(escapeCSVValue).join(','),
       ...rows.map((row) => row.join(',')),
     ].join('\n')
-    const blob = new Blob([csvContent], { type: 'text/csv;charset=utf-8;' })
+    const blob = new Blob(['\uFEFF' + csvContent], { type: 'text/csv;charset=utf-8;' })
     const url = window.URL.createObjectURL(blob)
     const link = document.createElement('a')
     link.href = url

frontend/src/views/user/__tests__/UsageView.spec.ts

@@ -237,6 +237,11 @@ describe('user UsageView', () => {
     }))
     expect(clickSpy).toHaveBeenCalled()
     expect(showSuccess).toHaveBeenCalled()
+    expect(csvContent.startsWith('\uFEFF')).toBe(true)
+    expect(csvContent.slice(1)).toBe([
+      'Time,API Key Name,Model,Reasoning Effort,Inbound Endpoint,IP Address,Type,Billing Mode,Input Tokens,Output Tokens,Cache Read Tokens,Cache Creation Tokens,Rate Multiplier,Billed Cost,Original Cost,First Token (ms),Duration (ms)',
+      '2026-03-08T00:00:00Z,demo-key,gpt-5.4,"\'-",,203.0.113.10,Sync,Token,4057,101,278272,4,1,0.09288300,0.09288300,12,345',
+    ].join('\n'))
     expect(csvContent).toContain('IP Address')
     expect(csvContent).toContain('203.0.113.10')
     expect(csvContent).toContain('Billed Cost')