fix: downgrade checkout/setup-node to v4 and make Vercel preview optional

[arnavkirti]

Description

This pull request updates all GitHub Actions workflows to use actions/checkout@v4 and actions/setup-node@v4 instead of v6, and makes the Vercel preview deployment workflow non-blocking and conditional on required secrets.

• Downgraded actions/checkout and actions/setup-node from v6 to v4 in build.yaml, labeler.yml, lint.yml, release.yml, security.yml, and vercel-preview.yml to align with supported versions and reduce unexpected CI breakages.

• Introduced a vercel-precheck job in vercel-preview.yml that verifies VERCEL_TOKEN, VERCEL_ORG_ID, and VERCEL_PROJECT_ID secrets before attempting a preview deploy.

• Made the deploy-preview job depend on vercel-precheck and only run when can_deploy is true, preventing failures when secrets are missing. Marking the Vercel deployment step as continue-on-error: true and summary note when failure.

Resolved or fixed issue

none

Affirmation

• My code follows the CONTRIBUTING.md guidelines

[vercel]

@arnavkirti is attempting to deploy a commit to the shopstr-eng Team on Vercel.

A member of the Team first needs to authorize it.

[GautamBytes]

LGTM Now!!