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/styles.css

@@ -52,6 +52,29 @@ html, body {
   background: #0c0906; opacity: 0; transition: opacity 0.18s ease;
 }
 
+/* 加载界面 */
+#loading {
+  position: fixed; inset: 0; z-index: 1000;
+  display: flex; align-items: center; justify-content: center;
+  background: #14100d;
+  transition: opacity 0.6s ease;
+}
+#loading.hidden { opacity: 0; pointer-events: none; }
+.loading-box { display: flex; flex-direction: column; align-items: center; gap: 18px; }
+.spinner {
+  width: 54px; height: 54px;
+  border: 3px solid rgba(201, 162, 39, 0.22);
+  border-top-color: #c9a227;
+  border-radius: 50%;
+  animation: spin 0.9s linear infinite;
+}
+@keyframes spin { to { transform: rotate(360deg); } }
+.loading-text {
+  font-family: var(--serif);
+  font-size: 14px; letter-spacing: 0.3em;
+  color: rgba(230, 216, 182, 0.7);
+}
+
 /* 顶部标题 */
 .hud { position: absolute; z-index: 5; }
 .top-left { top: 22px; left: 26px; }