Add export usage explainer

[JeremyZeng77]

Summary

• Adds a short "Using your export" explainer to the export options modal.
• Clarifies that the ZIP contains map.json and custom assets, can be re-imported into the editor, and can be used with a HYTOPIA SDK project.
• Explains when to enable bundled default asset options.

Addresses #158

Verification

• git diff --check
• npm run build (passes with existing repository warnings about missing mediapipe source map, lint warnings, and bundle size)

Bounty / payment

If this PR is accepted for the bounty/open-source contribution program, PayPal is preferred: https://paypal.me/Jeremytsangchina

Backup payout option: USDT TRC20 TWupTAoFjxR2VmqUUvzhfmUwXVZR6Kvz1Y

[vercel]

Someone is attempting to deploy a commit to the Hytopia Team on Vercel.

A member of the Team first needs to authorize it.

[JeremyZeng77]

Local verification completed for this change: \git diff --check\ passed and \
pm run build\ completed successfully. The Vercel status appears blocked by the team authorization page rather than by a build failure.