feat: Add decision limit selector with 5/10/20/50 options (#638)

## Summary
Allow users to select the number of decision records to display (5/10/20/50)
in the Web UI, with persistent storage in localStorage.

## Changes
### Backend
- api/server.go: Add 'limit' query parameter support to /api/decisions/latest
  - Default: 5 (maintains current behavior)
  - Max: 50 (prevents excessive data loading)
  - Fully backward compatible

### Frontend
- web/src/lib/api.ts: Update getLatestDecisions() to accept limit parameter
- web/src/pages/TraderDashboard.tsx:
  - Add decisionLimit state management with localStorage persistence
  - Add dropdown selector UI (5/10/20/50 options)
  - Pass limit to API calls and update SWR cache key

## Time Coverage
- 5 records = 15 minutes (default, quick check)
- 10 records = 30 minutes (short-term review)
- 20 records = 1 hour (medium-term analysis)
- 50 records = 2.5 hours (deep pattern analysis)

api/server.go

@@ -1448,7 +1448,15 @@ func (s *Server) handleLatestDecisions(c *gin.Context) {
 		return
 	}
 
-	records, err := trader.GetDecisionLogger().GetLatestRecords(5)
+	// 从 query 参数读取 limit，默认 5，最大 50
+	limit := 5
+	if limitStr := c.Query("limit"); limitStr != "" {
+		if l, err := strconv.Atoi(limitStr); err == nil && l > 0 && l <= 50 {
+			limit = l
+		}
+	}
+
+	records, err := trader.GetDecisionLogger().GetLatestRecords(limit)
 	if err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{
 			"error": fmt.Sprintf("获取决策日志失败: %v", err),