Fix frontend build cleanup on Windows

[snowyukitty]

What does this change?

Fixes #250.

Replace the POSIX-only rm executable in the workshop-frontend Vite+ clean:dist task with Node's built-in recursive filesystem removal. This lets the frontend build, and therefore pnpm lint, run on Windows without changing the task's cleanup behavior.

Why is this obviously correct and trivially verifiable?

fs.rmSync(..., { recursive: true, force: true }) has the two properties this task needs from rm -rf: it removes the complete dist tree and does not fail when the path is absent. Node is already the runtime for every repository task, so the replacement adds no dependency or platform-specific executable. The task remains uncached and remains the build dependency; only its command changes.

Verification on Windows 11 with Node 24.19.0 and pnpm 11.17.0:

• before: pnpm exec vp run -F @gadgets/workshop-frontend --no-cache build failed with Failed to find executable rm;
• after: the same command completed both TypeScript checks and the production Vite build;
• a stale sentinel placed under dist/ before the run was removed;
• pnpm lint completed successfully (existing warnings only).

AI tools assisted the investigation, patch drafting, and review. The failure and verification results above were reproduced locally.

Checklist

• This is a small, concrete change; it is not a feature, refactor, or low-value cleanup.
• I understand that maintainers decide whether the change is obviously correct and trivially verifiable.
• I have read and followed the contribution guidelines.