Add loading overlay while scenes initialize

- Show a spinner overlay that fades out once Cesium globe
  data is ready, with a 1s minimum and 8s timeout
- Expose a ready promise on CesiumGlobe resolved after country
  and city data load
- Remove temporary inspect script

index.html

@@ -63,6 +63,14 @@
     <div class="hud hint" id="hint">拖动旋转 · 滚轮缩放 · 时间轴视图拖拽左右滑动 · 点击选中 · ← → 翻页</div>
   </div>
 
+  <!-- 加载界面 -->
+  <div id="loading" aria-hidden="true">
+    <div class="loading-box">
+      <div class="spinner"></div>
+      <div class="loading-text">载入中…</div>
+    </div>
+  </div>
+
   <script type="module" src="/src/main.js"></script>
 </body>
 </html>