feat: add React cross-site events sample

[vystartasv]

Description

Adds a read-only React web part that aggregates Microsoft 365 group calendar views from a bounded set of SharePoint team-site sources.

What changed

• Added Graph calendarView integration through PnPjs Graph.
• Added bounded siteUrl|groupId source configuration.
• Added site URL validation and per-source result/error states.
• Added timezone-safe date normalization and display formatting.
• Added loading, empty, partial-failure, access-denied, retry, responsive, and accessible UI states.
• Added deterministic tests covering validation, date ranges, normalization, formatting, and error classification.
• Added README documenting Graph permissions, admin consent, tenant validation, and read-only scope.
• Added .nvmrc and valid PnP gallery metadata.

How was this tested?

• npm ci
• npm test — 10/10 tests passed across 5 suites.
• npm run build / gulp bundle --ship — TypeScript, Sass, lint, and webpack passed.
• gulp package-solution --ship — production .sppkg generated and validated.
• Reviewed the diff for scope, secrets, debug output, and generated artifacts.

Tenant validation requested

Graph calendar permissions, group/site correspondence, tenant consent, timezone behavior, and responsive rendering still require validation in a real SharePoint Online tenant. No tenant screenshots are fabricated.

[vystartasv]

Could someone with a SharePoint Online tenant please validate this sample with two representative Microsoft 365 team-site calendars and add real screenshots to assets/ or this PR if possible?

Please check Graph permission/admin-consent behavior, site URL/group ID pairing, calendarView results, time zones, partial failures, access-denied and retry states, and narrow responsive layouts. The implementation is read-only and does not create/update/delete events. Local tests, TypeScript, lint, webpack, and production packaging pass. Positive or negative findings are welcome. Thank you!

[github-actions]

Sample PR validation for #6481

⚠️ 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	⚠️ Warning
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.

[github-actions]

Sample PR validation for #6481

⚠️ 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.