feat: add React effective access audit sample

[vystartasv]

Description

Adds a read-only React Effective Access Audit web part for SharePoint. It displays direct role assignments on the current web and optionally a list, including principal type, login/email, role names, scope, and inheritance.

What changed

• Added safe site-relative path and list-title validation.
• Added bounded direct role-assignment loading through PnPjs.
• Added explicit limitation messaging: nested groups, sharing links, item/folder permissions, and transitive user access are not resolved.
• Added accessible Fluent UI table, loading, empty, error, and refresh states.
• Added dependency-free mapping/validation tests and gallery metadata.

How was this tested?

• npm install --package-lock-only --ignore-scripts
• npm ci
• npm test -- --clean — 5/5 tests passed.
• npm run build — TypeScript, ESLint, Sass, and webpack passed.
• npm run package — production .sppkg package generated and validated.
• Removed generated dependencies and build outputs before commit.

Tenant validation requested

Please validate with a SharePoint Online tenant and add a real screenshot if possible. In particular, check direct role-assignment visibility, inherited/unique scope labels, list-level access, and behavior when the signed-in user cannot read role assignments.

Type of change

• New sample
• Tests included
• Documentation included
• No fabricated tenant screenshots

[vystartasv]

Could someone with a SharePoint Online tenant please validate this sample with a representative web and list, then add a real screenshot to assets/ or this PR if possible?

The sample is intentionally read-only: it shows direct role assignments and clearly does not claim to expand nested groups or calculate a user’s complete transitive access. Local tests, TypeScript, ESLint, webpack, and production packaging pass. Tenant feedback would help confirm list-level behavior, inherited/unique labels, access-denied handling, and responsive presentation. Thank you!

[github-actions]

Sample PR validation for #6479

⚠️ Validation status: warnings

We automatically validate all pull requests against our contribution guidance to ensure that all samples provide a consistent experience to our community.

In order to merge this PR in a timely manner, the following criteria must be met:

Validation	Status
Pull request affects only one folder	✅ Succeeded
Sample folder name follows naming convention	✅ Succeeded
README.md contains visitor stat image	✅ Succeeded
Sample requires a .nvmrc file	✅ Succeeded
Sample requires a README.md	✅ Succeeded
Sample requires a screenshot .png in assets folder	⚠️ Warning
No .sppkg file	✅ Succeeded
No node_modules folder	✅ Succeeded
No lib folder	✅ Succeeded
No upgrade reports	✅ Succeeded

@vystartasv please address the above issues and push new changes to this branch.
For more information, see the contribution guidance.