refactor: tighten CLI helpers and sync lockfile

- merge.ts: surface the real tray icon error instead of assuming the
  path does not exist
- icon.ts: read the processed PNG into a buffer once and reuse it for
  the six Windows ICO resize frames instead of reopening the file
- ico.ts: drop exports from the ICO parsing internals so only
  writeIcoWithPreferredSize and buildIcoFromPngBuffers remain public
- pnpm-lock.yaml: record the @img/sharp-libvips-darwin-arm64 override
  added to package.json in 84cd1df