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)

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