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/scene/CesiumGlobe.js

@@ -69,6 +69,8 @@ export class CesiumGlobe {
     this.selected = -1;
     this._created = false;
     this._visible = false;
+    this._pendingData = 2; // 国家边界 + 城市数据
+    this.ready = new Promise((resolve) => { this._resolveReady = resolve; });
   }
 
   ensureCreated() {
@@ -316,6 +318,8 @@ export class CesiumGlobe {
       }
     } catch (err) {
       console.warn('[CesiumGlobe] 行政区划加载失败：', err);
+    } finally {
+      this._dataReady();
     }
   }
 
@@ -362,6 +366,15 @@ export class CesiumGlobe {
         },
       });
     }
+    this._dataReady();
+  }
+
+  _dataReady() {
+    this._pendingData--;
+    if (this._pendingData <= 0 && this._resolveReady) {
+      this._resolveReady();
+      this._resolveReady = null;
+    }
   }
 
   setSelected(index) {