fix: 推荐品种下拉改用缓存 main_code，避免加载卡住。

Co-authored-by: Cursor <cursoragent@cursor.com>

static/js/symbol.js

@@ -1,4 +1,35 @@
 (function () {
+    var recommendedGroupsCache = null;
+    var recommendedGroupsPromise = null;
+
+    function loadRecommendedGroups() {
+        if (recommendedGroupsCache) {
+            return Promise.resolve(recommendedGroupsCache);
+        }
+        if (recommendedGroupsPromise) {
+            return recommendedGroupsPromise;
+        }
+        recommendedGroupsPromise = fetch('/api/symbols/recommended')
+            .then(function (r) {
+                if (!r.ok) {
+                    throw new Error('HTTP ' + r.status);
+                }
+                return r.json();
+            })
+            .then(function (groups) {
+                recommendedGroupsCache = Array.isArray(groups) ? groups : [];
+                return recommendedGroupsCache;
+            })
+            .catch(function () {
+                recommendedGroupsCache = null;
+                throw new Error('load failed');
+            })
+            .finally(function () {
+                recommendedGroupsPromise = null;
+            });
+        return recommendedGroupsPromise;
+    }
+
     function formatSub(item) {
         var sub = '同花顺 ' + item.ths_code +
             (item.market_code ? ' · ' + item.market_code : '') +
@@ -47,7 +78,6 @@
         let abortCtrl = null;
         const cache = new Map();
         let mainsCache = null;
-        let mainsLoading = false;
 
         function hideDropdown() {
             dropdown.classList.remove('show');
@@ -135,16 +165,9 @@
                 renderGrouped(mainsCache, q);
                 return;
             }
-            if (mainsLoading) {
-                dropdown.innerHTML = '<div class="symbol-option">正在加载推荐品种…</div>';
-                dropdown.classList.add('show');
-                return;
-            }
-            mainsLoading = true;
             dropdown.innerHTML = '<div class="symbol-option">正在加载推荐品种…</div>';
             dropdown.classList.add('show');
-            fetch('/api/symbols/recommended')
-                .then(function (r) { return r.json(); })
+            loadRecommendedGroups()
                 .then(function (groups) {
                     mainsCache = groups;
                     if (!groups.length) {
@@ -156,10 +179,9 @@
                     showMarketMains(filterQ, onEmpty);
                 })
                 .catch(function () {
-                    hideDropdown();
-                })
-                .finally(function () {
-                    mainsLoading = false;
+                    dropdown.innerHTML =
+                        '<div class="symbol-option">推荐品种加载失败，请刷新页面或输入合约代码搜索</div>';
+                    dropdown.classList.add('show');
                 });
         }