fix: update darwin platform identifier to osx

[Chakyiu]

Overview

• Fixed: Failed to download OPKSSH binary due to mismatching OPKSSH binary found for platform darwin arm64

Changes Made

src/backend/utils/opkssh-binary-manager.ts

update darwin platform identifier to osx

Related Issues

None

Screenshots / Demos

Orignal Error Log:

[3:21:05 PM] [ERROR] [🚀] Failed to download OPKSSH binary [op:opkssh_binary_download_error]
Error: No matching OPKSSH binary found for platform darwin arm64
at OPKSSHBinaryManager.downloadBinary (file:///Termix/dist/backend/backend/utils/opkssh-binary-manager.js:49:23)
at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
at async OPKSSHBinaryManager.ensureBinary (file:///Termix/dist/backend/backend/utils/opkssh-binary-manager.js:38:13)
at async file:///Termix/dist/backend/backend/starter.js:102:13

Checklist

• Code follows project style guidelines
• Supports mobile and desktop UI/app (if applicable)
• I have read Contributing.md
• This is not a translation request. See docs