refactor: 整理资源文件，添加legacy路由

- 将原版文件移到frontend/legacy/3dearthmult/
- 纹理文件移到frontend/public/earth/assets/
- vite.config添加/legacy/earth路由支持
- earth.js纹理路径改为assets/

frontend/public/earth/js/earth.js

@@ -28,7 +28,7 @@ export function createEarth(scene) {
   scene.add(earth);
   
   const textureUrls = [
-    './8k_earth_daymap.jpg',
+    './assets/8k_earth_daymap.jpg',
     'https://raw.githubusercontent.com/mrdoob/three.js/dev/examples/textures/planets/earth_atmos_2048.jpg',
     'https://threejs.org/examples/textures/planets/earth_atmos_2048.jpg',
     'https://assets.codepen.io/982762/earth_texture_2048.jpg'