Add dark theme support via Qt Fusion palette

[nicks6240]

Summary

• Adds Dark theme checkbox under Options > Appearance (requires restart)
• Applies Qt Fusion style + a tuned dark \QPalette\ at startup when enabled
• Replaces all hardcoded colors in the log view paint path with \QPalette\ role lookups so they adapt automatically to both light and dark themes
• Setting persists as \�iew.darkTheme\ in the ini file; defaults to off so existing users are unaffected

Files changed

File	Change
\src/configuration.h/cpp\	Add \darkTheme_\ bool with accessors and persistence
\src/main.cpp\	Apply Fusion style + dark QPalette before MainWindow creation
\src/abstractlogview.cpp\	Replace hardcoded Qt::darkGray, Qt::lightGray, Qt::white, black/yellow QF highlight, lightyellow pull-to-follow with palette role lookups
\src/optionsdialog.ui/cpp\	Add Appearance group box with dark theme checkbox

Test plan

• Build with Qt5 + qmake and confirm no compile errors
• Launch without dark theme enabled - verify light theme looks identical to before
• Enable dark theme in Options, restart - verify dark UI throughout (menus, dialogs, log view, search bar)
• Open a log file and search to verify quick-find highlights are visible on dark background
• Toggle line numbers on/off and verify gutter/line-number area colors look correct in both themes