Add GitHub Pages web app manifest

[ClaudiuSchuster]

Summary

• add a root site.webmanifest for the GitHub Pages user site;
• connect it from index.html using the existing root favicon assets;
• document and verify the static Pages delivery path without adding .htaccess, a service worker, tracking, or external manifest resources.

Verification

• git diff --check
• documented image and local HTTP checks from docs/README.md
• manifest JSON, field, icon-reference, and local application/manifest+json checks
• live pre-change Pages baseline: root source is main; site.webmanifest currently returns 404

GitHub Pages publishes this repository root directly from main, so the manifest is included as an ordinary static root asset.