style(public/index.css): 调整背景按钮和切换开关的样式

统一背景按钮和本地背景切换开关的尺寸、内边距与布局，优化视觉和交互样式细节

public/index.css

@@ -305,21 +305,31 @@ body {
   z-index: 1000;
 }
 
+.bg-btn-wrapper .bg-local-switch {
+  flex-direction: row;
+  height: 28px;
+  gap: 0;
+}
+
+.bg-btn-wrapper .bg-local-switch input {
+  top: 0;
+}
+
 /* 换背景按钮（仅图标） */
 .change-background-btn {
   background: transparent;
   border: none;
   border-radius: 50%;
-  width: auto;
-  height: auto;
-  padding: 2px;
+  width: 28px;
+  height: 28px;
+  padding: 0;
   color: var(--text-light);
-  font-size: 1.6rem;
+  font-size: 1.2rem;
   line-height: 1;
   cursor: pointer;
   transition: all 0.2s ease;
   backdrop-filter: none;
-  display: inline-flex;
+  display: flex;
   align-items: center;
   justify-content: center;
 }
@@ -328,7 +338,6 @@ body {
   background: rgba(255, 255, 255, 0.2);
   outline: 1px solid rgba(255, 255, 255, 0.3);
   backdrop-filter: blur(10px);
-  padding: 2px;
 }
 
 .change-background-btn::after {