add standalone exe file.

Self-contained Standalone EXE version: No requirements (PyInstaller packs a python interpreter runtime into the .exe so no need to install python separately)

Once automatically packed into a self-contained Standalone EXE by the ubiquitous PyInstaller, Windows Defender falsely flags it as containing malware. If you're uncomfortable with that, perhaps download (and review) the source .py and run that instead.
Or, perhaps, to verify that the FolderCompareSync.exe you downloaded matches the published github auto-built Release and has not been tampered with, you may choose to follow these steps:

• Download from the Release: FolderCompareSync.exe and FolderCompareSync.exe.sha512
• Open Command Prompt in the folder containing these files. and run the following command to compute the SHA512 hash of the EXE: certutil -hashfile FolderCompareSync.exe SHA512
• Compare the output hash with the contents of the FolderCompareSync.exe.sha512 file. They should match exactly. If the SHA512 hashes do not match, do not run the executable as it may have been tampered with.

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Standalone EXE build details (CI)
• Python 3.13 + PyInstaller (optimized)
• Bundled python modules: tzdata, python-dateutil, tkinter
• A SHA512 checksum is attached (FolderCompareSync.exe.sha512) for verification of github auto-built Standalone EXE