fix(issue-7): revert dev artifact and fix test timeouts

- Revert next.config.ts: remove hardcoded Cloudflare tunnel URL from
  allowedDevOrigins (dev artifact, not part of the voice upload fix).
- Rewrite voice transcribe tests to use mock request objects instead of
  real Request/FormData/Blob instances that cause formData() to hang
  indefinitely in vitest's Node environment. All 9 tests now pass.

Made-with: Cursor