feat: 添加设置入口并统一背景样式

新增设置菜单入口，将其放在侧边栏底部系统信息区下方
统一背景绘制到 html 元素上，避免元素高度差导致的留白
临时下架背景选项 bg-3，调整侧边栏布局与样式
新增中英文的设置翻译文本，优化系统信息区的间距

src/layout/Layout.vue

@@ -1,6 +1,6 @@
 <template>
-  <!-- 应用整体：背景（渐变 / 图片） + 悬浮面板 -->
-  <div class="app-shell flex h-screen w-screen flex-col overflow-hidden" :style="shellStyle">
+  <!-- 应用整体：背景由 <html> 统一绘制，此处只负责布局 -->
+  <div class="app-shell flex h-full w-full flex-col overflow-hidden">
     <!-- 顶部自定义 TitleBar -->
     <Header @settings="goToSetting" />
 
@@ -18,46 +18,11 @@
 <script setup lang="ts">
 import Header from "./header/Header.vue"
 import Aside from "./aside/Aside.vue"
-import { useAppStore } from "@/stores/app"
 
 const router = useRouter()
-const appStore = useAppStore()
 
 /** 点击标题栏“设置”按钮 → 跳转设置页 */
 function goToSetting() {
   router.push("/setting")
 }
-
-/**
- * 选中背景图片时覆盖默认渐变背景。
- * `cover` 让图片按原始比例缩放铺满窗口，不会变形拉伸。
- */
-const shellStyle = computed(() =>
-  appStore.backgroundUrl
-    ? {
-        backgroundImage: `url('${appStore.backgroundUrl}')`,
-        backgroundSize: "cover",
-        backgroundPosition: "center",
-        backgroundRepeat: "no-repeat",
-      }
-    : {},
-)
 </script>
-
-<style scoped lang="scss">
-.app-shell {
-  background-color: #eef2ff;
-  background-image:
-    radial-gradient(1200px 600px at 10% -10%, rgba(99, 102, 241, 0.28), transparent 60%),
-    radial-gradient(1000px 500px at 110% 10%, rgba(34, 211, 238, 0.28), transparent 55%),
-    linear-gradient(160deg, #eef2ff 0%, #f8fafc 45%, #eef7ff 100%);
-}
-
-.dark .app-shell {
-  background-color: #0b1220;
-  background-image:
-    radial-gradient(1200px 600px at 10% -10%, rgba(99, 102, 241, 0.35), transparent 60%),
-    radial-gradient(1000px 500px at 110% 10%, rgba(14, 165, 233, 0.28), transparent 55%),
-    linear-gradient(160deg, #0b1220 0%, #0f172a 45%, #111827 100%);
-}
-</style>