Fix dark inactive tab title visibility

[Leoncl2025]

Summary

• prevent Qt Advanced Docking System from reapplying its internal stylesheet after a Light-to-Dark palette change
• preserve Notepad # active and inactive tab title colors in Dark mode
• cover both inactive titles in a three-tab Light-to-Dark regression test

Validation

• Release build with Visual Studio 2022 / MSVC 19.44 and Qt 6.5.3
• focused darkStyleKeepsInactiveTabTitlesReadable regression test
• full CTest suite: 4/4 passed