fix: adapt frontend image paths for backend port 8002 and use blob download

Backend port changed from 8000 to 8002 due to port conflict (frpc on Windows).
- Add resolveImageUrl() to convert relative image URLs to full backend URLs
- Fix all spatial page image src and download handlers to use backend address
- Fix chat message image rendering to use correct port
- Switch download to blob-based approach to support cross-origin file saving

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

web/src/lib/api.ts

@@ -12,7 +12,14 @@ import type {
 } from "@/types";
 
 // API基础配置
-const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || (typeof window !== 'undefined' ? `${window.location.protocol}//${window.location.hostname}:8000` : "http://127.0.0.1:8000");
+const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || (typeof window !== 'undefined' ? `${window.location.protocol}//${window.location.hostname}:8002` : "http://127.0.0.1:8002");
+
+// 将后端返回的相对路径图片URL转为完整的后端地址
+export function resolveImageUrl(url: string): string {
+  if (!url) return url;
+  if (url.startsWith("http://") || url.startsWith("https://") || url.startsWith("data:")) return url;
+  return `${API_BASE_URL}${url.startsWith("/") ? "" : "/"}${url}`;
+}
 
 // 请求拦截器
 async function apiRequest<T>(