[Bug] Artifact sharing not working in Claude Cowork integration

Description:
Artifacts fail to display and share after creation. Two errors occur together:

1. [CoworkArtifacts] ERR_ABORTED (-3) loading cowork-artifact://local/{id}/index.html immediately after artifact creation — the protocol handler registers but the content isn't ready to serve yet (race condition in showArtifact).
2. [CoworkArtifacts] share_from_content failed → HTTP 404 not_found_error from Anthropic API — because the WebContents never loaded, there's nothing to upload when sharing.

Affected artifacts: desk-attendance, hubspot-pulse, my-open-hubspot-tickets

Platform: macOS darwin, v2.1.148

Reference: Feedback ID 26dcd506-f517-44c1-a380-f22439f4b7db, log lines ~52313–52452 and ~53290–53370 in main.log