Modernize GUI Appearance

[MarcusStill]

Adds a QSS stylesheet (files/style.qss) implementing a modern Windows 10/11 light theme (Segoe UI, flat design, #0078D4 accent). Updates main.py to load this theme safely using pathlib with a fallback try-except block. Also updates sys.exit(app.exec()) to properly capture the exit code, following Loguru best practices.

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PR created automatically by Jules for task 18130861925171657665 started by @MarcusStill

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