Add standalone mode for HTML artifact downloads

When downloading HTML artifacts, the generated HTML now works standalone
without requiring the extension runtime.

Changes:
- Add PrepareHtmlOptions config object to SandboxedIframe.prepareHtmlDocument()
- Add isStandalone flag to skip runtime bridge and navigation interceptor
- When isStandalone=true:
  - window.sendRuntimeMessage is NOT defined
  - Navigation interceptor is NOT injected
  - Artifact runtime providers fall back to embedded data
- HtmlArtifact download button now uses isStandalone: true

This fixes the issue where downloaded HTML artifacts would:
- Try to call window.sendRuntimeMessage (which would fail silently)
- Try to postMessage to non-existent parent window
- Not work when opened via file:// protocol

Now downloaded artifacts work completely standalone with embedded data.

packages/web-ui/src/tools/artifacts/HtmlArtifact.ts

@@ -48,7 +48,10 @@ export class HtmlArtifact extends ArtifactElement {
 		const sandbox = this.sandboxIframeRef.value;
 		const sandboxId = `artifact-${this.filename}`;
 		const downloadContent =
-			sandbox?.prepareHtmlDocument(sandboxId, this._content, this.runtimeProviders || [], true) || this._content;
+			sandbox?.prepareHtmlDocument(sandboxId, this._content, this.runtimeProviders || [], {
+				isHtmlArtifact: true,
+				isStandalone: true, // Skip runtime bridge and navigation interceptor for standalone downloads
+			}) || this._content;
 
 		return html`
 			<div class="flex items-center gap-2">