feat(home): 页面底色改紫色渐变铺满

整页背景由近白 #fafafb 换为薰衣草紫渐变
(#ede7fd → #d8defc)，与卡片和渐变描边更统一。

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

frontend/src/views/HomeView.vue

@@ -334,13 +334,13 @@ async function copyQqGroup() {
   display: flex;
   flex-direction: column;
 }
-/* 平铺浅色底盖掉全局 aurora 配色，与 vivid --bg: #fafafb 一致 */
+/* 整页紫色渐变底铺满 */
 .home::before {
   content: '';
   position: fixed;
   inset: 0;
   z-index: -1;
-  background: #fafafb;
+  background: linear-gradient(160deg, #ede7fd 0%, #e2d9fb 45%, #dcd7fa 75%, #d8defc 100%);
   pointer-events: none;
 }