build(scripts): 优化预览图生成逻辑和压缩参数

1. 调整预览图目标尺寸为4096宽按比例缩放，减小文件体积避免加载失败
2. 增加目标尺寸日志输出方便调试
3. 提升png压缩等级并启用自适应滤波优化图片质量和大小

scripts/gen-preview.cjs

@@ -19,9 +19,11 @@ async function main() {
   const h = image.getHeight()
   console.log('TIFF size:', w, 'x', h)
 
-  // Target size matching current preview
-  const targetW = 6181
-  const targetH = 8192
+  // 控制生成纹理尺寸：原图 14997x19876 太大，浏览器加载 123MB PNG 容易失败。
+  // 4096 宽按原比例缩放后约 5425 高，文件体积降到 1/4 左右，仍然足够清晰。
+  const targetW = 4096
+  const targetH = Math.round(targetW * h / w)
+  console.log('Target size:', targetW, 'x', targetH)
 
   console.log('Reading rasters at target size...')
   const data = await image.readRasters({ width: targetW, height: targetH, samples: [0, 1, 2] })
@@ -44,7 +46,7 @@ async function main() {
   console.log('Black pixels masked:', blackCount)
 
   await sharp(rgba, { raw: { width: targetW, height: targetH, channels: 4 } })
-    .png({ compressionLevel: 6 })
+    .png({ compressionLevel: 9, adaptiveFiltering: true })
     .toFile(outPath)
 
   console.log('Saved:', outPath)