experimental: SSG download support (#3783)

## Description

See corresponding [SaaS
PR](https://github.com/webstudio-is/webstudio-saas/pull/311) too

## Steps for reproduction

Click Publish/Export

<img width="363" alt="image"
src="https://github.com/user-attachments/assets/fbc1f4e5-3a2c-4bb9-9242-025196f991bc">


Deploy zip to netlify
<img width="1710" alt="image"
src="https://github.com/user-attachments/assets/daca4ead-6272-4729-8e82-0986301e5c8b">


## Code Review

- [ ] hi @kof, I need you to do
  - conceptual review (architecture, feature-correctness)
  - detailed review (read every line)
  - test it on preview

## Before requesting a review

- [ ] made a self-review
- [ ] added inline comments where things may be not obvious (the "why",
not "what")

## Before merging

- [ ] tested locally and on preview environment (preview dev login:
5de6)
- [ ] updated [test
cases](https://github.com/webstudio-is/webstudio/blob/main/apps/builder/docs/test-cases.md)
document
- [ ] added tests
- [ ] if any new env variables are added, added them to `.env` file

packages/cli/src/prebuild.ts

@@ -296,11 +296,6 @@ export const prebuild = async (options: {
     );
   }
 
-  const domain = siteData.build.deployment?.projectDomain;
-  if (domain === undefined) {
-    throw new Error(`Project domain is missing from the project data`);
-  }
-
   // collect all possible component metas
   const metas = new Map<string, WsComponentMeta>();
   const componentSources = new Map<string, string>();
@@ -448,6 +443,15 @@ export const prebuild = async (options: {
   const assetsToDownload: Promise<void>[] = [];
 
   const appDomain = options.preview ? "wstd.work" : "wstd.io";
+
+  const domain =
+    siteData.build.deployment?.destination === "static"
+      ? siteData.build.deployment?.assetsDomain
+      : siteData.build.deployment?.projectDomain;
+  if (domain === undefined) {
+    throw new Error(`Project domain is missing from the project data`);
+  }
+
   const assetBuildUrl = `https://${domain}.${appDomain}/cgi/asset/`;
 
   const imageLoader = createImageLoader({

packages/cli/templates/ssg/app/constants.mjs

@@ -9,5 +9,9 @@ export const imageBaseUrl = "/assets/";
  * @type {import("@webstudio-is/image").ImageLoader}
  */
 export const imageLoader = ({ src }) => {
+  if (URL.canParse(src)) {
+    return src;
+  }
+
   return imageBaseUrl + src;
 };