feat: add React KPI scorecard sample

[vystartasv]

Description

Adds a read-only React KPI scorecard web part for SharePoint lists. It displays up to six configured KPIs with current value, target, threshold state, trend, and last observation date.

What changed

• Added configurable SharePoint list and field mapping.
• Added safe validation for internal field names, root paths, date filters, and bounded queries.
• Added deterministic threshold, delta, trend, and value-normalization utilities.
• Added Fluent UI cards with loading, empty, error, refresh, and accessible status states.
• Added README setup instructions and gallery metadata.

How was this tested?

• npm ci
• npm test -- --clean — 5/5 tests passed.
• npm run build — TypeScript, ESLint, Sass, and webpack passed with zero warnings.
• npm run package — production SharePoint package generated and validated successfully.
• Removed generated dependencies and build artifacts before commit.

Tenant validation requested

This sample needs SharePoint Online tenant validation with a representative KPI list. Please add a real tenant screenshot showing the configured web part and report any compatibility issues.

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 using a representative KPI list and add a real screenshot to assets/ or this PR?

The local build, tests, lint, and production package pass, but tenant validation is intentionally still needed for configuration UX, list-field compatibility, threshold rendering, and responsive behavior. Any findings—positive or negative—would be very helpful. Thank you!

[github-actions]

Sample PR validation for #6478

⚠️ 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.