fix(fields): set readOnly on virtual fields in admin UI

[howwohmm]

Problem

Virtual fields (virtual: true) are computed server-side and never persisted to the database. Despite this, the admin UI renders them as standard editable inputs — users can type values into them, but those values are silently discarded on save.

Fix

During field sanitization in sanitizeField, after field.admin is guaranteed to exist, set readOnly: true on any field where virtual === true:

if ('virtual' in field && field.virtual === true && !field.admin.readOnly) {
  field.admin.readOnly = true
}

The guard (!field.admin.readOnly) preserves explicit readOnly: false overrides in case a developer has a reason to display a virtual field differently.

Scope

1 file, 6-line addition in packages/payload/src/fields/config/sanitize.ts. No behavior change for non-virtual fields.

Fixes #16013

[github-actions]

Pull Request titles must follow the Conventional Commits specification and have valid scopes.

Unknown scope "fields" found in pull request title "fix(fields): set readOnly on virtual fields in admin UI". Scope must match one of: cpa, claude, db-*, db-d1-sqlite, db-mongodb, db-postgres, db-vercel-postgres, db-sqlite, db-d1-sqlite, drizzle, email-*, email-nodemailer, email-resend, eslint, graphql, kv, kv-redis, live-preview, live-preview-react, live-preview-vue, next, payload-cloud, plugin-cloud, plugin-cloud-storage, plugin-ecommerce, plugin-form-builder, plugin-import-export, plugin-mcp, plugin-multi-tenant, plugin-nested-docs, plugin-redirects, plugin-search, plugin-sentry, plugin-seo, plugin-stripe, richtext-*, richtext-lexical, richtext-slate, sdk, storage-*, storage-azure, storage-gcs, storage-r2, storage-uploadthing, storage-vercel-blob, storage-s3, translations, ui, templates, examples(/(\w|-)+)?, deps.

feat(ui): add Button component
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[PatrikKozak]

Hey @howwohmm, thank you for the PR!

We really appreciate you taking the time to work on Payload with us but I'm going to close this PR as there is already a very similar PR open for this exact fix here: #16016