Feat/sticky header gradient experimental studio

[omareltomy]

• Closes

Additional details

Steps to test

How has the user experience changed?

PR Tasks

• Have tests been added/updated?
• Has a PR for user-facing changes been opened in cypress-documentation?
• Have API changes been updated in the type definitions?

---

Note

Makes the runnable header sticky with a gradient and wires the experimentalStudio flag from window config to toggle it.

• Reporter UI:
  • runnable-header: Adds sticky-header variant with sticky positioning and gradient separator.
• Prop plumbing:
  • header.tsx/runnable-header.tsx: Pass and consume experimentalStudio to enable sticky header.
  • main.tsx: Reads window.__CYPRESS_CONFIG__.base64Config, decodes JSON, and passes experimentalStudio into Header via renderReporterHeader.

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[jennifer-shehane]

@omareltomy Thanks for the contribution! Could you please sign our CLA?