Add Database page with new connection feature

- Reorganize project structure: move frontend to web/ directory
- Add Database page with connection list (name, type, subtables, status, created, actions)
- Integrate Element Plus for UI components with dark theme support
- Add Quicksand font for rounded UI design
- Configure root package.json to run frontend from web/ directory

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/App.vue

@@ -0,0 +1,41 @@
+<script setup lang="ts">
+import { computed } from 'vue'
+import { useRoute } from 'vue-router'
+import { ElConfigProvider } from 'element-plus'
+import Sidebar from '@/components/Sidebar.vue'
+
+const route = useRoute()
+const showSidebar = computed(() => route.path !== '/')
+</script>
+
+<template>
+  <ElConfigProvider :z-index="3000">
+    <div class="flex min-h-screen bg-dark-900">
+      <!-- 左侧侧边栏 (除了登录页) -->
+      <Sidebar v-if="showSidebar" />
+
+      <!-- 右侧内容区 -->
+      <main :class="showSidebar ? 'ml-64' : ''" class="flex-1 min-h-screen">
+        <router-view class="page-content" />
+      </main>
+    </div>
+  </ElConfigProvider>
+</template>
+
+<style>
+/* 页面进入动画 */
+.page-content {
+  animation: page-enter 0.3s ease-out;
+}
+
+@keyframes page-enter {
+  from {
+    opacity: 0;
+    transform: translateY(10px);
+  }
+  to {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}
+</style>