feat(fullstack): support custom base URLs

[Copilot]

• Closes [fullstack] support custom base #1260

Asset URLs were hardcoded with "/" prefix, preventing use of custom base paths or dynamic URL generation via renderBuiltUrl callback.

Changes

Ported asset URL generation logic from @vitejs/plugin-rsc:

• RuntimeAsset class - Represents dynamic runtime expressions that bypass JSON stringification
• serializeValueWithRuntime() - Serializes manifests while preserving runtime expressions as code
• assetsURL() - Generates asset URLs respecting config.experimental.renderBuiltUrl callback, falling back to config.base + url
• writeAssetsManifest() - Uses assetsURL() instead of hardcoded "/" for all asset paths

Example

Custom base configuration:

// vite.config.ts
export default defineConfig({
  base: '/my-app/',
  // Asset URLs now use: /my-app/assets/chunk-abc123.js
})

Dynamic URL generation:

// vite.config.ts
export default defineConfig({
  experimental: {
    renderBuiltUrl(filename) {
      return { runtime: `__CDN_BASE__ + ${JSON.stringify(filename)}` }
    }
  }
  // Manifest contains: window.__CDN_BASE__ + "assets/chunk-abc123.js"
})

Original prompt

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This section details on the original issue you should resolve

<issue_title>[fullstack] support custom base</issue_title>
<issue_description>

vite-plugins/packages/fullstack/src/plugin.ts

Lines 370 to 379 in ac7707d

	// TODO: base
	if (environmentName === "client") {
	result.entry = `/${chunk.fileName}`;
	result.js = deps.js.map((fileName) => ({
	href: `/${fileName}`,
	}));
	}
	result.css = deps.css.map((fileName) => ({
	href: `/${fileName}`,
	}));

We should be able to port this logic from @vitejs/plugin-rsc https://github.com/vitejs/vite-plugin-react/blob/3506cfa5ab086b90b8c96878f385d77762bd277c/packages/plugin-rsc/src/plugin.ts#L1793-L1822</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [fullstack] support custom base #1260

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