feat: 新增设置页面及外观设置功能

添加了设置页面路由、外观设置子页面，支持主题模式切换和自定义窗口背景图片，新增相关 Rust 后端模块、数据库初始化、多语言文案及配置项，更新 Tauri 配置和项目描述。

src/layout/Layout.vue

@@ -1,8 +1,8 @@
 <template>
-  <!-- 应用整体：背景渐变 + 悬浮面板 -->
-  <div class="app-shell flex h-screen w-screen flex-col overflow-hidden">
+  <!-- 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">
     <!-- 顶部自定义 TitleBar -->
-    <Header @settings="settingsOpen = true" />
+    <Header @settings="goToSetting" />
 
     <!-- 主体：侧边栏 + 内容区 -->
     <div class="flex min-h-0 flex-1 gap-3 px-3 pb-3">
@@ -12,38 +12,50 @@
         <router-view />
       </main>
     </div>
-
-    <!-- 设置（占位） -->
-    <a-modal
-      v-model:open="settingsOpen"
-      :title="t('settings.title')"
-      :footer="null"
-      centered
-    >
-      <a-result status="info" :sub-title="t('settings.placeholder')" />
-    </a-modal>
   </div>
 </template>
 
 <script setup lang="ts">
-import { useI18n } from "vue-i18n"
 import Header from "./header/Header.vue"
 import Aside from "./aside/Aside.vue"
+import { useAppStore } from "@/stores/app"
 
-const { t } = useI18n()
-const settingsOpen = ref(false)
+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:
+  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:
+  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%);