Add failing test and potential fix for Underlinenav overflow issue

[siddharthkp]

• Adds playwright mcp server
• Adds a page in examples/nextjs app to reproduce https://github.com/github/primer/issues/6291
• Hides overflow on SSR only to enable it on client

[changeset-bot]

⚠️ No Changeset found

Latest commit: 1273fa0

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[github-actions]

👋 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. Or, apply the integration-tests: skipped manually label to skip these checks.

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/11688

[primer-integration]

Integration test results from github/github-ui:

  CI Passed
  VRT Passed
  Projects Passed

All checks passed!