fix(core): remove unused g_windowDpi variable

[cmixed]

g_windowDpi was assigned in RefreshWindowDpi() but never read anywhere. The actual DPI scaling is driven by g_uiScale via ApplyUIScale(), making g_windowDpi dead code since its introduction in d93f544.

part of information in bash :

main.cpp(584,13): error: variable 'g_windowDpi' set but not used [-Werror,-Wunused-but-set-global]
  584 | static UINT g_windowDpi = USER_DEFAULT_SCREEN_DPI;
      |             ^
1 error generated.

QuickView> cmake --build out/build/Release-LTO
[2/2] Linking CXX executable QuickView.exe