fix: 大幅增强SRTM高程数据地形可见性

问题：
- 上传SRTM数据后预览仍然看不到明显地形起伏
- 原因：SRTM 90米分辨率数据高差相对平缓，需要更强的视觉增强

修复内容：
1. 垂直夸张系数从2.0增加到5.0倍（针对SRTM等低分辨率数据）
2. 增强光照：添加定向光源，强度2.0，优化阴影对比度
3. 优化相机视角：pitch从-0.6改为-1.0（更倾斜），距离从2.4x改为2.0x（更近）
4. 更新用户提示文字，说明使用了5倍垂直夸张

技术细节：
- DirectionalLight方向(0.2, 0.5, -0.8)产生侧光效果，增强地形阴影
- 更陡的相机角度让用户能更清楚地看到起伏
- 更近的距离让90米分辨率的细节更可见

影响范围：
- 用户现在可以清晰看到SRTM数据的地形起伏
- 山脉、山谷、坡度变化都有明显的视觉效果

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/components/elevation-preview-cesium-map.tsx

@@ -150,6 +150,11 @@ export function ElevationPreviewCesiumMap({
         viewer.scene.globe.baseColor = Cesium.Color.fromCssColorString("#0f172a");
         // Enable lighting to enhance terrain visibility with shadows
         viewer.scene.globe.enableLighting = true;
+        // Increase lighting intensity for better terrain shadow contrast
+        viewer.scene.light = new Cesium.DirectionalLight({
+          direction: new Cesium.Cartesian3(0.2, 0.5, -0.8),
+          intensity: 2.0,
+        });
         viewer.scene.backgroundColor = Cesium.Color.fromCssColorString("#020617");
         viewer.scene.screenSpaceCameraController.enableCollisionDetection = true;
         const creditContainer = viewer.cesiumWidget.creditContainer as HTMLElement | null;
@@ -214,8 +219,9 @@ export function ElevationPreviewCesiumMap({
       }
 
       setTerrainError("");
-      // Increase vertical exaggeration to make terrain relief more visible
-      viewer.scene.verticalExaggeration = 2.0;
+      // Significantly increase vertical exaggeration for SRTM data (90m resolution)
+      // SRTM data often has subtle elevation changes that need strong exaggeration
+      viewer.scene.verticalExaggeration = 5.0;
       viewer.scene.verticalExaggerationRelativeHeight = 0.0;
 
       // Remove all existing imagery layers first
@@ -445,7 +451,9 @@ export function ElevationPreviewCesiumMap({
       const boundingSphere = Cesium.BoundingSphere.fromPoints(positions);
       void viewer.camera.flyToBoundingSphere(boundingSphere, {
         duration: 0.8,
-        offset: new Cesium.HeadingPitchRange(0, -0.6, Math.max(1200, boundingSphere.radius * 2.4)),
+        // Use steeper pitch angle (-1.0) to better show terrain relief
+        // Closer distance (2.0x instead of 2.4x) to see more detail
+        offset: new Cesium.HeadingPitchRange(0, -1.0, Math.max(1200, boundingSphere.radius * 2.0)),
       });
     }
   }, [altitudeRange.max, altitudeRange.min, dataset, ready, safeCells, safePoints, terrainError, terrainRenderState]);
@@ -467,7 +475,7 @@ export function ElevationPreviewCesiumMap({
   return (
     <div className="space-y-2">
       <div className="text-xs text-slate-500">
-        颜色由蓝到红表示高程由低到高；地形瓦片就绪时优先加载真实三维地形，失败时自动回退到现有色带/点位预览。
+        颜色由蓝到红表示高程由低到高；地形瓦片就绪时优先加载真实三维地形（垂直夸张5倍以增强可见性），失败时自动回退到现有色带/点位预览。
       </div>
       {terrainError ? (
         <Alert