feat(export): show selected count on batch export button

src/pages/ExportPage.scss

@@ -1145,14 +1145,27 @@
       font-size: 12px;
       cursor: pointer;
       margin-left: auto;
+      display: inline-flex;
+      align-items: center;
+      gap: 6px;
 
       &:hover:not(:disabled) {
         background: var(--primary-hover);
       }
 
-      &:disabled {
-        opacity: 0.65;
-        cursor: not-allowed;
+      .selection-export-count {
+        min-width: 18px;
+        height: 18px;
+        padding: 0 5px;
+        border-radius: 999px;
+        background: rgba(255, 255, 255, 0.2);
+        color: #fff;
+        font-size: 11px;
+        font-weight: 700;
+        display: inline-flex;
+        align-items: center;
+        justify-content: center;
+        font-variant-numeric: tabular-nums;
       }
     }
   }

src/pages/ExportPage.tsx

@@ -3984,14 +3984,16 @@ function ExportPage() {
                   >
                     清空
                   </button>
+                  {selectedCount > 0 && (
                     <button
                       className="selection-export-btn"
                       type="button"
                       onClick={openBatchExport}
-                    disabled={selectedCount === 0}
                     >
-                    批量导出
+                      <span>批量导出</span>
+                      <span className="selection-export-count">{selectedCount}</span>
                     </button>
+                  )}
                 </div>
                 <div className="contacts-list-header">
                   <span className="contacts-list-header-select">选择</span>