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

src/main.js

@@ -8,3 +8,16 @@ for (const ev of ['copy', 'cut', 'contextmenu']) {
 // 启动应用
 const app = new App();
 window.__app = app; // 便于调试
+
+// 加载界面：等三个视图初始化完成（最少展示 1 秒，最多等 8 秒）后淡出
+const t0 = Date.now();
+const MIN_DISPLAY = 1000;
+Promise.race([
+  app.globe.ready,
+  new Promise((r) => setTimeout(r, 8000)),
+]).then(() => {
+  const wait = Math.max(0, MIN_DISPLAY - (Date.now() - t0));
+  setTimeout(() => {
+    document.getElementById('loading').classList.add('hidden');
+  }, wait);
+});