style: 调整通知图标颜色为浅白色

- 改为rgba(255, 255, 255, 0.75)浅白色，与系统设置图标颜色一致
- 悬停时改为rgba(255, 255, 255, 0.95)更亮的白色

frontend/src/components/NotificationBell.vue

@@ -261,14 +261,14 @@ onUnmounted(() => {
   display: flex;
   align-items: center;
   justify-content: center;
-  color: $text-secondary;
+  color: rgba(255, 255, 255, 0.75);
   cursor: pointer;
   position: relative;
   transition: all $transition-base $ease-out;
   
   &:hover {
     background: $bg-card;
-    color: $text-primary;
+    color: rgba(255, 255, 255, 0.95);
   }
 }