refactor: 统计页面改名为"总览"并提升至侧边栏首位

将侧边栏"用量统计"菜单项改名为"总览"，移至第一位，
默认路由重定向从 /providers 改为 /stats

frontend/src/__tests__/components/AppLayout.test.tsx

@@ -31,7 +31,7 @@ describe('AppLayout', () => {
     renderWithRouter(<AppLayout />)
 
     expect(screen.getAllByText('供应商管理').length).toBeGreaterThan(0)
-    expect(screen.getAllByText('用量统计').length).toBeGreaterThan(0)
+    expect(screen.getAllByText('总览').length).toBeGreaterThan(0)
   })
 
   it('renders settings menu item', () => {