Docs: show the bare Stackyard mark above the README title

[SandObserver]

Summary

• Replace the tiled app icon above the README title with the bare Stackyard mark.
• Serve the white mark (docs/logo.svg) on dark themes and the dark mark (docs/logo-light.svg) on light themes through a <picture> element.

Testing
All green.

Changelog

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.