Fix light rendering of grids and Settings dialog in dark mode

[Hirogen]

Dark mode (Application.SetColorMode) remaps SystemColors, but anything painted by the Windows visual-style renderer only exists in a light flavor. Two symptoms of that plus some hardcoded light-mode colors:

• Grid column headers stayed light — EnableHeadersVisualStyles was left at its default. New PaintHelper.ApplyGridViewTheme disables it in dark mode; wired via PaintHelper.SetColumnizer so all grids get it (main, filter, Bookmark window, Pattern window), plus the three grids in the Settings dialog.
• Settings dialog tab pages were light — UseVisualStyleBackColor = true paints the page body light while child controls inherit the dark ambient color. New PaintHelper.ApplyTabControlTheme flips it off in dark mode.
• Hardcoded paint fallbacks — cells for lines matching no highlight rule got white background/black text regardless of theme; the unfocused-selection brush was a fixed light gray; GetDataGridViewCellStyle had white-on-white ForeColor. All PaintHelper color helpers now take an explicit darkMode parameter (call sites pass Application.IsDarkModeEnabled), keeping them pure and unit-testable.

18 new tests in PaintHelperTests. Light-mode behavior unchanged.