[Runner] Make tray icon context menu theme aware

[Knyrps]

Summary

Makes the PowerToys runner's system tray icon right-click context menu honor the OS dark / light theme. Previously the popup menu was always rendered in the default light Win32 style regardless of the system theme.

Closes #31813

Detail

• Adds a small native helper DarkMode under src/common/Themes/dark_mode.{h,cpp} that loads the undocumented uxtheme.dll ordinals shipped on Windows 10 1903+ / Windows 11:
  • 132 ShouldAppsUseDarkMode
  • 135 SetPreferredAppMode (applied as ForceDark / ForceLight)
  • 136 FlushMenuThemes
  • ApplyToMenu(HMENU) sets MIM_BACKGROUND with MIM_APPLYTOSUBMENUS so the popup and any future submenus pick up the right background brush.
• Wires it into src/runner/tray_icon.cpp:
  • DarkMode::Initialize() once at the end of start_tray_icon.
  • DarkMode::ApplyToMenu(h_menu) inside the WM_RBUTTONUP / WM_CONTEXTMENU branch, immediately before TrackPopupMenu, so the theme is re-evaluated every time the menu opens.
  • DarkMode::Refresh() from the existing handle_theme_change so a live OS theme switch is picked up without restarting PowerToys.
• Precedent: this is the same approach already used by ZoomIt — see src/modules/ZoomIt/ZoomIt/Utility.cpp (InitializeDarkModeSupport, ApplyDarkModeToMenu). The helper is intentionally minimal and only exposes what the tray menu needs.

Screenshots

Before: white menu background on a dark Windows theme (per issue #31813).

After: menu background matches the OS theme; tested live in both dark and light mode with a runtime theme switch (no restart required).

How tested

• MSBuild src\common\Themes\Themes.vcxproj /p:Configuration=Release /p:Platform=x64 /m /restore — green, 0 warnings, 0 errors.
• MSBuild src\runner\runner.vcxproj /p:Configuration=Release /p:Platform=x64 /m /restore — green, 0 warnings, 0 errors.
• Ran the freshly-built src\runner\x64\Release\PowerToys.exe:
  • System in dark mode → right-clicked tray icon → context menu rendered with dark background.
  • Switched Windows to light mode → re-opened menu → rendered with light background, no restart needed.