feat: add React intranet announcements notification center

[vystartasv]

Description

Adds a bounded, read-only React Intranet Announcements and Targeted Notification Center web part for SPFx 1.23.2.

What changed

• Reads configured SharePoint announcement lists with REST GET only.
• Applies explicit publish and expiry date eligibility rules using a configurable reference date.
• Provides configured audience-label matching without resolving group membership or making authorization decisions.
• Supports bounded four-source, five-page, 50-row-page, 200-row-per-source reads.
• Validates server-relative paths, managed paths, same-tenant HTTPS/root-relative links, images, and OData next links.
• Handles malformed rows, missing optional fields, empty/partial results, access errors, throttling, retry information, and loading states.
• Includes accessible responsive cards, search/category filtering, status messaging, and keyboard-operable controls.
• Does not send, create, update, dismiss, or delete notifications and does not modify SharePoint data.

Validation

• npm ci
• npm test — 7 tests passed.
• npm run build — SPFx bundle passed.
• npm run package — production .sppkg passed.
• npm run verify — read-only verifier passed after generated-artifact cleanup.
• Metadata, configuration, engine, scope, whitespace, and generated-artifact checks passed.

Tenant validation requested

Please validate representative SharePoint Online announcement lists and real tenant data, including future/expired dates, missing dates, published flags, configured audience labels, missing optional fields, managed paths, unsafe and valid links/images, malformed rows, paging bounds, empty results, partial failures, permission denial, throttling, keyboard access, and responsive layout. Real tenant screenshots are welcome; none are fabricated locally.

[vystartasv]

Could someone with a SharePoint Online tenant please validate PR #6491 and add real screenshots if possible?

Please test representative announcement-list data covering future/expired dates, missing dates, published flags, configured audience labels, optional fields, managed paths, valid and unsafe links/images, malformed rows, paging bounds, empty results, partial failures, permission denial, throttling, keyboard access, and responsive layout.

This is a bounded presentation/review aid using SharePoint REST GET requests only. It never sends, creates, updates, dismisses, or deletes notifications. Audience matching is label comparison only; it does not resolve Microsoft 365 group membership or authorize access. No screenshots are fabricated locally.

[github-actions]

Sample PR validation for #6491

⚠️ 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	⚠️ Warning
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 #6491

⚠️ 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.