feat(web): trade detail modal with prices, fees, timestamps

- Click any trade row in history table to open detail modal
- Modal shows 6 sections: 概览, 时间, 价差, 手续费, 多仓, 空仓
- Entries and exits displayed with 6 decimal precision
- Fee entry/exit and total fee displayed
- Open/close timestamps with full date-time format
- Duration, scale count, total amount, exit reason
- Orders sub-table if available
- Escape key and overlay click to close

trader.go

@@ -119,7 +119,10 @@ type Trader struct {
 	mu            sync.Mutex
 	positions     map[string]*ArbPosition // coin -> position
 	lastTradeTime map[string]time.Time
-	closedTrades  []TradeRecord // history of closed trades
+	closedTrades  []TradeRecord // history of closed trades (current session)
+
+	// Historical stats loaded from DB on startup — combined with session stats in GetClosedStats
+	dbConverged, dbDiverged, dbFlat, dbTotal int
 
 	OnTradeEvent func(event string, data interface{}) // P3-4: real-time SSE push
 
@@ -163,6 +166,10 @@ func NewTrader(cfg *Config, database *db.DB) *Trader {
 	// Restore open positions from DB on restart
 	if database != nil {
 		t.restoreOpenPositions()
+		// Load historical closed trade stats for convergence display
+		if c, d, f, tot, err := database.GetClosedStats(); err == nil {
+			t.dbConverged, t.dbDiverged, t.dbFlat, t.dbTotal = c, d, f, tot
+		}
 	}
 
 	return t
@@ -720,10 +727,13 @@ func weightedAvgPrice(prices []float64, amountPerTrade float64) float64 {
 	return totalCost / totalShares
 }
 
-// GetClosedStats returns convergence stats from all closed trades.
+// GetClosedStats returns convergence stats from all closed trades (DB history + current session).
 func (t *Trader) GetClosedStats() (converged, diverged, flat, total int) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
+	// Start with DB historical counts
+	converged, diverged, flat, total = t.dbConverged, t.dbDiverged, t.dbFlat, t.dbTotal
+	// Add in-memory session trades
 	for _, tr := range t.closedTrades {
 		total++
 		switch tr.Convergence {