feat: Add System Overview tab to User Management page (admin only)

- Add backend API endpoint /api/users/system-strategies for system-wide strategy data
- Query all strategies across all users with positions, PnL, trade stats
- Add summary statistics (total strategies, running count, total capital, total PnL/ROI)
- Support filtering by status (running/stopped) and search by strategy/symbol/user
- Add System Overview tab with summary cards and detailed strategy table
- Display user, strategy name, status, symbol, capital, PnL/ROI, positions, trades, indicator, exchange, timeframe, leverage
- Add i18n translations for zh-CN and en-US
- Lazy-load strategy data when tab is first accessed

quantdinger_vue/src/api/user.js

@@ -208,3 +208,15 @@ export function getUserCreditsLog (params) {
     params
   })
 }
+
+/**
+ * Get system-wide strategy overview (admin only)
+ * @param {Object} params - { page, page_size, status, search }
+ */
+export function getSystemStrategies (params) {
+  return request({
+    url: '/api/users/system-strategies',
+    method: 'get',
+    params
+  })
+}