feat: Modernize PySide6 GUI with Windows 10/11 QSS styling

[MarcusStill]

Introduced files/style.qss to provide a flat, modern Windows 10/11 look (Segoe UI, #0078D4 accent color) without adding heavy third-party libraries. Modified main.py to load the stylesheet securely using an absolute path.

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PR created automatically by Jules for task 2211782384264779652 started by @MarcusStill

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