CI: run next lint (and tests) in the frontend job

[Psalmuel01]

The CI workflow (.github/workflows/ci.yml) frontend job runs only pnpm tsc --noEmit and pnpm build. It does not run next lint, and (once tests exist) will not run them. Lint errors and test failures reach main uncaught.

Scope

• Add pnpm lint (next lint) as a required step in the frontend CI job.
• Add pnpm test once the Vitest setup lands (gate this on the SDK/API test issues).
• Keep the existing typecheck + build steps.
• Fail the job on lint/test failure.

Acceptance

• Frontend CI runs lint + typecheck + test + build; a lint error or failing test blocks the merge.

✅ Merge requirements (read before starting)

Before this PR can be merged, all of the following must hold:

• All CI checks pass (contracts, frontend, circuits as applicable)
• Every Greptile review comment is addressed — no unresolved review threads
• Greptile confidence score is 4/5 or higher

Greptile performs the detailed code review on each PR. Please resolve its feedback fully before requesting a human merge — PRs below 4/5 or with open Greptile threads will not be merged.