refactor: move FatalError to form-component lib

[Magnusrm]

Description

• Migrates FatalError and FatalErrorEmpty from src/App/frontend/src/app-components/error to libs/form-component/src/app-components/FatalError, and updates all 10 consumers in src/App/frontend to import from @app/form-component.
• Adds Vitest unit tests and a Storybook story.

Verification

• Related issues are connected (if applicable)
• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)

Summary by CodeRabbit

Release Notes

• Tests

  • Added comprehensive test coverage for fatal error component and empty error state handling.

• Documentation

  • Added design documentation with visual previews for fatal error states.

• Chores

  • Improved component organisation and consolidated error components within the form library.
  • Code cleanup and optimised import structure across components.

[coderabbitai]

📝 Walkthrough

Walkthrough

This pull request relocates the FatalError and FatalErrorEmpty error components from frontend-specific paths into the shared @app/form-component library. The components undergo minor refactoring (removing unused React imports, streamlining JSX), receive comprehensive test coverage and Storybook documentation, and all consuming frontend files are updated to import from the new shared location.

Changes

FatalError component library migration

Layer / File(s)	Summary
FatalError component implementation and cleanup libs/form-component/src/app-components/FatalError/FatalError.tsx, libs/form-component/src/app-components/FatalError/FatalErrorEmpty.tsx	FatalError and FatalErrorEmpty components drop unused React default imports and streamline JSX formatting whilst preserving the same rendered structure and prop handling.
FatalError test coverage libs/form-component/src/app-components/FatalError/FatalError.test.tsx	React Testing Library tests verify FatalError renders children within a data-fatal-error wrapper, forwards arbitrary props to the underlying div, and FatalErrorEmpty renders as a hidden empty div.
FatalError module exports and library integration libs/form-component/src/app-components/FatalError/index.ts, libs/form-component/src/app-components/index.ts	FatalError module index establishes public re-exports and the form-component barrel export includes the module in the public API surface.
FatalError Storybook stories libs/form-component/src/app-components/FatalError/FatalError.stories.tsx	Storybook metadata and two stories document FatalError rendering with default children and FatalErrorEmpty as a hidden marker element.
Frontend consumer import migration src/App/frontend/src/components/altinnError.tsx, src/App/frontend/src/layout/GenericComponent.tsx, src/App/frontend/src/layout/SigneeList/SigneeListError.tsx, src/App/frontend/src/layout/SigneeList/SigneeListSummary.tsx, src/App/frontend/src/layout/SigningActions/SigningActionsComponent.tsx, src/App/frontend/src/layout/SigningDocumentList/SigningDocumentListError.tsx, src/App/frontend/src/layout/Subform/SubformComponent.tsx, src/App/frontend/src/layout/Subform/Summary/SubformSummaryComponent.tsx, src/App/frontend/src/layout/Subform/Summary/SubformSummaryTable.tsx	Twelve frontend files update their FatalError imports to use @app/form-component instead of relative src/app-components paths, consolidating error UI imports with other form components.
Monorepo changed-paths documentation src/App/frontend/monorepo-changed-paths.txt	The tracked paths file removes a settings file reference, documents the relocated FatalError and FatalErrorEmpty components, and adds the new InstantiationError.tsx file.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested labels

skip-releasenotes, kind/chore

Suggested reviewers

• adamhaeger
• framitdavid
• phlipsterit

Poem

A rabbit hops through error paths,
Moving components left and right,
From scattered src/ to shared lib's way—
Now FatalError shines so bright! 🐰✨

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'refactor: move FatalError to form-component lib' accurately and clearly summarises the main change: migrating the FatalError component to a shared library.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The pull request description is complete and follows the repository template with all required sections filled.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch refactor/move-error-to-app

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[JamalAlabdullah]

Looks good! 👍