Add social preview image for repository links

[PSU3D0]

Context

Ticket: 702-github-repo-housekeeping

GitHub social previews currently use a generic card. We should upload a project-branded social preview image (1280x640) to improve link previews on Slack/X/Discord/Reddit.

Acceptance criteria

• Create a 1280x640 social preview image for Formualizer
• Include concise value props (e.g. Rust + Python + WASM, 320+ functions, Arrow-powered)
• Upload it in repository settings so link cards render the new preview
• Add source assets to the repo assets directory (PNG + editable source if available)