Add Windows dark mode with classic light default

[Leoncl2025]

Summary

• Add live System, Light, and Dark appearance modes across the Windows application.
• Theme Qt controls, Scintilla surfaces, syntax styles, docks, dialogs, search, and Compare using first-party semantic tokens inspired by VS Code Dark 2026.
• Keep classic Light as the default for fresh installs; invalid persisted values still fall back safely to System.

Implementation notes

• No QDarkStyleSheet or new runtime dependency.
• User custom.css remains the final stylesheet override.
• Includes Windows title-bar integration and dynamic system appearance tracking.

Testing

• Built NotepadSharp, AppearanceManagerTests, and EditorAppearanceTests with VS 2022 / MSVC 19.44 and Qt 6.5.3.
• Full CTest suite: 4/4 passed.
• AppearanceManager QtTest cases: 9/9 passed.
• git diff --check main...HEAD passed.

Reference: dail8859/NotepadNext#407