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/ai/src/agent/agent-loop.ts

@@ -61,7 +61,7 @@ export function agentLoop(
 				queuedMessages = [];
 			}
 
-			console.log("agent-loop: ", [...currentContext.messages]);
+			// console.log("agent-loop: ", [...currentContext.messages]);
 
 			// Stream assistant response
 			const message = await streamAssistantResponse(currentContext, config, signal, stream, streamFn);