fix: navigate to slug based project url on buy page

[blushi]

Description

Redirect to slug based project url if project has a slug.
Doing the same as on the project page so that it retrieves slug-based project on sanity.

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Author Checklist

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please add links to any relevant follow up issues.

I have...

• provided a link to the relevant issue or specification
• provided instructions on how to test
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

How to test

https://deploy-preview-2507--regen-marketplace.netlify.app/project/C01-001/buy should redirect to https://deploy-preview-2507--regen-marketplace.netlify.app/project/mai-ndombe-4/buy

Reviewers Checklist

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• confirmed all author checklist items have been addressed
• reviewed code correctness and readability
• verified React components follow DRY principles
• reviewed documentation is accurate
• reviewed tests
• manually tested (if applicable)

[netlify]

❌ Deploy Preview for regen-website failed. Why did it fail? →

Name	Link
🔨 Latest commit	f41d072
🔍 Latest deploy log	https://app.netlify.com/sites/regen-website/deploys/671753e48ab6a100086912de

[blushi]

@erikalogie see testing instructions

[erikalogie]

LGTM