docs(assets): update repository favicon and icon assets

[AndresSaa]

What and why

Update repository favicon and icon branding using the new SVG source (.github/assets/wal-icon.svg) and add multi-resolution PNG and ICO assets (16x16, 24x24, 32x32, 48x48, 64x64, 128x128, 180x180, 256x256, 512x512) for full compatibility across browsers, documentation tooling, and external websites.

Testing

Validated formatting, linting, and package sanity:

• npm run lint passed with 0 errors
• npm test passed (69/69 tests)
• run lint:package passed (publint + attw + smoke tests)

Checklist

• PR title is a valid Conventional Commit (it becomes the squash commit on main)
• CI is green on the full Node × OS matrix
• Any new durability claim has a test in this PR (see AGENTS.md → contract invariants)
• User-visible changes update CHANGELOG.md ([Unreleased])
• No AI attribution anywhere in commits, this PR body, or code comments