- 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
TIANHE
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