ci: add GitHub Actions workflow for contracts and frontend

[Jaydbrown]

Closes #16

Summary

• Contracts job: format check → Clippy → cargo test → WASM build (wasm32-unknown-unknown)
• Frontend job: pnpm install → pnpm -r run typecheck → pnpm --filter frontend lint → pnpm build
• Both jobs run on every push and pull request to main

Test plan

• CI fails on a broken Rust test
• CI fails on a type error or lint error in the frontend
• CI fails on a build break

[vercel]

@Jaydbrown is attempting to deploy a commit to the Bukunmi-Dev Projects Team on Vercel.

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