feat: add DropZone and App layout skeleton

src/App.vue

@@ -1,9 +1,48 @@
 <script setup>
+import { useCalendar } from './composables/useCalendar.js'
+import DropZone from './components/DropZone.vue'
+
+const { isLoaded } = useCalendar()
 </script>
 
 <template>
-  <div style="font-family: sans-serif; padding: 40px;">
-    <h1>📅 ICS 日程整理器</h1>
-    <p>脚手架就绪</p>
+  <div class="app">
+    <header class="app-header">
+      <h1>📅 ICS 日程整理器</h1>
+    </header>
+    <main class="app-main">
+      <DropZone v-if="!isLoaded" />
+      <div v-else class="placeholder">
+        <p>文件已加载（列表与编辑器待实现）</p>
+      </div>
+    </main>
   </div>
 </template>
+
+<style>
+* { box-sizing: border-box; margin: 0; padding: 0; }
+body {
+  font-family: -apple-system, "Segoe UI", Roboto, "Microsoft YaHei", sans-serif;
+  background: #f5f6f8;
+  color: #222;
+  line-height: 1.5;
+}
+.app { min-height: 100vh; display: flex; flex-direction: column; }
+.app-header {
+  background: #2c3e50;
+  color: #fff;
+  padding: 14px 24px;
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+}
+.app-header h1 { font-size: 18px; font-weight: 600; }
+.app-main { flex: 1; display: flex; overflow: hidden; }
+.placeholder {
+  flex: 1;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  color: #95a5a6;
+}
+</style>