Reduce Blankslate small size padding

[Copilot]

Closes https://github.com/github/primer/issues/6240

Blankslate size="small" did not meaningfully reduce padding for dense layouts. This updates size padding so small is visibly tighter while medium and large use the requested default spacing.

Changelog

New

Changed

• Blankslate spacing
  • small: var(--base-size-16)
  • medium / large: var(--base-size-24)

<Blankslate size="small">...</Blankslate>

Removed

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Review the small, medium, and large Blankslate variants to confirm the updated padding scale feels balanced in compact containers.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 42856c6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

To publish a canary release for integration testing, apply the Canary Release label to this PR.

[Copilot]

Pull request overview

Adjusts @primer/react’s Blankslate spacing scale so size="small" is meaningfully denser, while medium/large use a tighter default padding. This fits into the component styling system by updating the CSS module tokens and adding a regression test + changeset for the consumer-facing styling change.

Changes:

• Update Blankslate padding tokens: small → --base-size-16, medium/large → --base-size-24.
• Add a unit test that asserts the CSS variable values for each size variant.
• Refactor Storybook Vite host allowlist configuration to use a shared constant (and add a core.allowedHosts entry).

Show a summary per file

File	Description
packages/styled-react/.storybook/main.ts	Extracts a shared STORYBOOK_ALLOWED_HOSTS constant and applies it to Vite server config (and adds core.allowedHosts).
packages/react/src/Blankslate/Blankslate.test.tsx	Adds a stylesheet-rule based test asserting --blankslate-padding values for small/medium/large.
packages/react/src/Blankslate/Blankslate.module.css	Updates --blankslate-padding for size variants and adds a container-query override for small.
packages/react/.storybook/main.ts	Same Storybook allowed-hosts refactor as styled-react Storybook config (and adds core.allowedHosts).
package-lock.json	Updates locked workspace/version metadata to match the current package versions.
.changeset/tidy-blankslate-padding.md	Adds a patch changeset for the consumer-facing Blankslate spacing adjustment.

Copilot's findings

• Files reviewed: 5/72 changed files
• Comments generated: 3

[primer-integration]

Integration test results from github/github-ui PR:

  CI Passed
  VRT skipped
  Projects Passed

VRT check ensures that when visual differences are detected, the PR cannot proceed until someone acknowledges the changes by adding the "visual difference acknowledged" label.

Need help? If you believe this failure is unrelated to your changes, please reach out to the Primer team for assistance.