占位图生成
生成纯色 / 渐变占位图，可下载 PNG / SVG，支持自定义文字与配色。
本地处理
<svg xmlns="http://www.w3.org/2000/svg" width="600" height="400" viewBox="0 0 600 400"><rect width="600" height="400" fill="#e2e8f0"/><text x="50%" y="50%" font-family="-apple-system, BlinkMacSystemFont, 'Segoe UI', 'PingFang SC', 'Hiragino Sans GB', 'Microsoft YaHei', sans-serif" font-size="50" font-weight="600" fill="#475569" text-anchor="middle" dominant-baseline="central">600 × 400</text></svg>data:image/svg+xml;utf8,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%22600%22%20height%3D%22400%22%20viewBox%3D%220%200%20600%20400%22%3E%3Crect%20width%3D%22600%22%20height%3D%22400%22%20fill%3D%22%23e2e8f0%22%2F%3E%3Ctext%20x%3D%2250%25%22%20y%3D%2250%25%22%20font-family%3D%22-apple-system%2C%20BlinkMacSystemFont%2C%20'Segoe%20UI'%2C%20'PingFang%20SC'%2C%20'Hiragino%20Sans%20GB'%2C%20'Microsoft%20YaHei'%2C%20sans-serif%22%20font-size%3D%2250%22%20font-weight%3D%22600%22%20fill%3D%22%23475569%22%20text-anchor%3D%22middle%22%20dominant-baseline%3D%22central%22%3E600%20%C3%97%20400%3C%2Ftext%3E%3C%2Fsvg%3ECSS：
background-image: …在浏览器本地用 SVG 生成，PNG 由 Canvas 实时栅格化下载。无网络请求，无水印。