Infra: CI for contract tests + frontend checks

[Omoboi-dev]

Ensure GitHub Actions runs `cargo test` for contracts and `pnpm -r typecheck` / `pnpm --filter frontend lint` / `pnpm build` on PRs.

Acceptance criteria

• CI fails on a broken test, type error, lint error, or build break.
• Badge/status reflected in the README.

[Jaydbrown]

I would like to work on this.