docs(readme): show the Flagward mark above the title

[basb7]

What does this change?

The README opened straight into # Flagward with no visual mark. Now the same frontend/public/logo.png the dashboard serves sits centered above the title at its natural 76px width, referenced by relative path so it renders on GitHub.

Same accessibility call as the nav and auth pages: empty alt, because the H1 right below already reads "Flagward" — a labelled mark would announce the name twice.

How was it verified?

Markdown-only change; no code touched. Rendered path resolves to the file shipped in #61 (frontend/public/logo.png).

Checklist

• Tests added or updated for this change
• ruff check . passes
• pytest passes
• npm run lint and npm run build pass, if the frontend changed
• Documentation updated, if behaviour changed
• Commits follow Conventional Commits

Backend/frontend boxes are checked on the strength that nothing outside README.md changed — CI will confirm. Docs box unchecked: the README is the change.