feat(web,engine): custom style — image upload, AI-extract prompt, painter ref

自定义画风入口里加上传按钮：客户端把图缩到 512px webp(base64)，传到新
路由 /api/parse-style-image，vision LLM 解析成英文 style prompt 回填 textarea；
图本身随 sessionStorage → /api/start → Session.styleReferenceImage 透传，
painter.collectReferenceImages 把它置于 slot 0，整局每一幕都作为 reference
图锚定画风（brush / color / mood），比 priorScene 优先级更高。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

lib/ai-client/index.ts

@@ -1,5 +1,5 @@
 export { chat } from "./chat";
 export { generateImage } from "./image";
 export type { GenerateImageOptions, GenerateImageResult } from "./image";
-export { interpretClick } from "./vision";
+export { interpretClick, analyzeImageDataUrl } from "./vision";
 export type { ChatMessage } from "./chat";