docs: defer the "No Figma link" hint until the entry is prepared

[frankieyan]

Short description

This follow up fix to #1076 and #1080 eliminates a flash of the "No Figma link" badge on stories/docs pages off of a fresh page load. It does so by leveraging Storybook's getCurrentStoryData().prepared value off of its Context

Test plan

• Run Storybook locally. In the steps below, don't just rely on the client-side routing, but reload each page instead
• Navigate to a story that has a Figma link defined (e.g. Button) and reload
  • No "No Figma link" badge flashes before the Figma button appears
• Navigate to a docs page with a Figma link (e.g. Avatar docs) and reload
  • No badge flash there either
• Open a story/component with no figma param (e.g. Box) and reload
  • The "No Figma link" badge is rendered
• Open a FIGMA_NOT_NEEDED story (e.g. KeyCapturer) and reload
  • Neither the badge nor the button are shown
• Open an unattached-MDX docs page (e.g. a Tips & tricks page)
  • Neither the badge nor the button are shown

Demo

First load of a linked story (e.g. Button):

Before	After
Screen.Recording.2026-06-29.at.7.40.53.PM.mov	Screen.Recording.2026-06-29.at.7.46.46.PM.mov

PR Checklist

• Added tests for bugs / new features
• Updated docs
• Reviewed and approved Chromatic visual regression tests in CI

[doistbot]

This PR fixes the "No Figma link" badge flash on fresh page loads by gating the badge on Storybook's prepared flag, and updates the plop template to re-export FIGMA_NOT_NEEDED.

Few things worth tightening:

• Exporting FIGMA_NOT_NEEDED from the ./figma barrel pulls a preview-side constant through an entrypoint that already re-exports FigmaTool, which depends on Storybook's manager API. Consider keeping story/preview consumers on a preview-safe path like ./constants or a dedicated preview export instead of widening this barrel.

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