Fix Windows dev startup

[neubig]

Summary

• launch the frontend npm script from dev-safe through npm's own CLI path when available, with OS-specific fallbacks for direct runs
• repin @openhands/typescript-client to a commit that includes the Windows rimraf dist install fix from Make windows install work through rimraf typescript-client#137
• make the dev-safe tests path-portable and cover the npm command selection paths
• unblock local verification on the current toolchain with the TypeScript 6 deprecation setting and a narrower WebSocket send type

Verification

• npm ci
• npx vitest run __tests__/scripts/dev-safe.test.ts
• npm run typecheck
• npm run build
• npm run dev smoke reached the frontend dev server at http://localhost:3001/