Improved theme validation modals

[ErisDS]

Summary

• Makes theme validation outcomes clearer by separating blocking fatal GScan issues from secondary non-blocking issues.
• Updates failed upload/save/activation modals to show blocking issues first, and successful upload/save/activation modals to keep optional compatibility issues secondary.
• Adds a shared validation details disclosure and Storybook spike coverage for fatal/success scenarios without changing GScan levels, validation behavior, or API data shape.

Implementation notes

• Uses problem.fatal === true as the blocking discriminator.
• Keeps non-fatal errors/warnings/recommendations in a “non-blocking issues” disclosure.
• Defaults secondary details to collapsed outside development and expanded in development.
• Adds sticky footer support and ReactNode titles to the Admin-X confirmation modal shell so long validation content does not hide modal actions.
• Keeps the Shade Storybook spike as a visual reference for scenario coverage.

Screenshots

Scenario	Before	After
Fatal only
Fatal with non-blocking issues
Success only
Success with non-blocking issues

Development mode

Secondary details are expanded by default in development.

Testing

• CI=true pnpm install using Node 22.22.2
• pnpm --filter @tryghost/shade lint using Node 22.22.2
  • Passes with an existing unrelated filters.tsx warning.
• pnpm --filter @tryghost/admin-x-settings lint using Node 22.22.2
  • Passes with existing unrelated any warnings.
• pnpm --filter @tryghost/admin-x-settings build using Node 22.22.2
  • Passes with an existing package export warning.
• pnpm --filter @tryghost/admin-x-design-system lint using Node 22.22.2
• pnpm --filter @tryghost/admin-x-design-system build using Node 22.22.2

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