fix: replace hardcoded Windows path separators with os.sep (#34)

[tejassinghbhati]

Closes #34

Summary

Replaces all hardcoded Windows-style backslash path separators ('\\') with os.sep from the Python standard library in the S3 upload logic.

This is a non-breaking fix that advances the project's stated goal of platform-independence.

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Changes

API/Classes/Base/SyncS3.py

• uploadSync()
  • Default prefix parameter changed from '\\' to os.sep
• uploadSync()
  • fileName.replace('\\', '/') → fileName.replace(os.sep, '/')
• updateSync()
  • localFile.replace('\\', '/') → localFile.replace(os.sep, '/')

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API/Routes/Upload/UploadRoute.py

• upload_dir()
  • Default prefix parameter changed from '\\' to os.sep
• upload_dir()
  • fileName.replace('\\', '/') → fileName.replace(os.sep, '/')

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Notes

• os is already imported in both files — zero new dependencies introduced.
• os.sep evaluates to '\\' on Windows, so existing Windows behaviour is completely unchanged.
• On macOS and Linux, os.sep correctly evaluates to '/', fixing silent failures in S3 path construction on those platforms.
• Directly addresses the constraint listed in docs/ARCHITECTURE.md:

"Hardcoded or relative path assumptions exist and reduce portability."

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Testing

• Verified that os.sep resolves to '\\' on Windows (behaviour preserved).
• Verified that logic using startswith(prefix) and replace(prefix, "", 1) behaves identically when prefix=os.sep.
• Confirmed correct S3 key generation on macOS/Linux environments.
• Confirmed no regression in existing upload functionality.

[SeaCelo]

Retargeted to mac-port per the macOS integration track: #51.
We are validating Mac/cross-platform changes in mac-port first, then merging mac-port -> main.