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),

web/src/lib/api.ts

@@ -261,12 +261,21 @@ export const api = {
     return res.json()
   },
 
-  // 获取最新决策（支持trader_id）
+  // 获取最新决策（支持trader_id和limit参数）
-  async getLatestDecisions(traderId?: string): Promise<DecisionRecord[]> {
+  async getLatestDecisions(
-    const url = traderId
+    traderId?: string,
-      ? `${API_BASE}/decisions/latest?trader_id=${traderId}`
+    limit: number = 5
-      : `${API_BASE}/decisions/latest`
+  ): Promise<DecisionRecord[]> {
-    const res = await httpClient.get(url, getAuthHeaders())
+    const params = new URLSearchParams()
+    if (traderId) {
+      params.append('trader_id', traderId)
+    }
+    params.append('limit', limit.toString())
+
+    const res = await httpClient.get(
+      `${API_BASE}/decisions/latest?${params}`,
+      getAuthHeaders()
+    )
     if (!res.ok) throw new Error('获取最新决策失败')
     return res.json()
   },

web/src/pages/TraderDashboard.tsx

@@ -54,6 +54,18 @@ export default function TraderDashboard() {
   )
   const [lastUpdate, setLastUpdate] = useState<string>('--:--:--')
 
+  // 决策记录数量选择（从 localStorage 读取，默认 5）
+  const [decisionLimit, setDecisionLimit] = useState<number>(() => {
+    const saved = localStorage.getItem('decisionLimit')
+    return saved ? parseInt(saved, 10) : 5
+  })
+
+  // 当 limit 变化时保存到 localStorage
+  const handleLimitChange = (newLimit: number) => {
+    setDecisionLimit(newLimit)
+    localStorage.setItem('decisionLimit', newLimit.toString())
+  }
+
   // 获取trader列表（仅在用户登录时）
   const { data: traders, error: tradersError } = useSWR<TraderInfo[]>(
     user && token ? 'traders' : null,
@@ -111,8 +123,10 @@ export default function TraderDashboard() {
   )
 
   const { data: decisions } = useSWR<DecisionRecord[]>(
-    selectedTraderId ? `decisions/latest-${selectedTraderId}` : null,
+    selectedTraderId
-    () => api.getLatestDecisions(selectedTraderId),
+      ? `decisions/latest-${selectedTraderId}-${decisionLimit}`
+      : null,
+    () => api.getLatestDecisions(selectedTraderId, decisionLimit),
     {
       refreshInterval: 30000,
       revalidateOnFocus: false,
@@ -570,9 +584,10 @@ export default function TraderDashboard() {
           style={{ animationDelay: '0.2s' }}
         >
           <div
-            className="flex items-center gap-3 mb-5 pb-4 border-b"
+            className="flex items-center justify-between mb-5 pb-4 border-b"
             style={{ borderColor: '#2B3139' }}
           >
+            <div className="flex items-center gap-3">
               <div
                 className="w-10 h-10 rounded-xl flex items-center justify-center"
                 style={{
@@ -594,6 +609,32 @@ export default function TraderDashboard() {
               </div>
             </div>
 
+            {/* 显示数量选择器 */}
+            <div className="flex items-center gap-2">
+              <span className="text-xs" style={{ color: '#848E9C' }}>
+                {language === 'zh' ? '显示' : 'Show'}:
+              </span>
+              <select
+                value={decisionLimit}
+                onChange={(e) => handleLimitChange(parseInt(e.target.value, 10))}
+                className="rounded px-2 py-1 text-xs font-medium cursor-pointer transition-colors"
+                style={{
+                  background: '#1E2329',
+                  border: '1px solid #2B3139',
+                  color: '#EAECEF',
+                }}
+              >
+                <option value={5}>5</option>
+                <option value={10}>10</option>
+                <option value={20}>20</option>
+                <option value={50}>50</option>
+              </select>
+              <span className="text-xs" style={{ color: '#848E9C' }}>
+                {language === 'zh' ? '条' : ''}
+              </span>
+            </div>
+          </div>
+
           <div
             className="space-y-4 overflow-y-auto pr-2"
             style={{ maxHeight: 'calc(100vh - 280px)' }}