[Obsidian] Handle Expired Account Errors

[rasulkireev]

Summary

• Detect the structured expired-account response from the Obsidian API: error: "account_expired".
• Show the API-provided message in the normal Obsidian notice/status UI, with a small fallback message if the response omits one.
• Keep the UX intentionally small: no modal and no extra upgrade link.
• Move response error parsing into a small testable helper and add focused tests for the account-expired behavior.
• Prepare release 3.0.3 by updating package.json, package-lock.json, manifest.json, versions.json, and the plugin client version constant.

Backend dependency

This expects the backend response from readwiseio/rekindled#9358:

{
  "error": "account_expired",
  "message": "...",
  "upgrade_url": "..."
}

Release

After this PR merges, create and push tag 3.0.3 to run the existing release workflow. The workflow builds the plugin and creates a draft GitHub release with main.js, manifest.json, and styles.css.

Validation

• npm test
• npm run ci

The build passes. It still prints the existing non-fatal dotenv preventAssignment warning.