Add social preview image for repository links #7
Description
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)