Fix ImGui DPI scaling by using DPI-scaled fonts and removing inverse global scaling

[alxbilger]

This PR fixes incorrect ImGui font scaling on high-DPI monitors (e.g. 4K), where text appeared significantly smaller than intended.

Previously, the code applied DPI scaling twice in opposite directions:

• Font sizes were multiplied by the monitor content scale (yscale)
• ImGuiIO::FontGlobalScale was then divided by the same DPI factor

This effectively canceled the DPI scaling, causing text to render at near-non-DPI-aware sizes on high-resolution displays.

I also reload the font when changing the content scale (for example when moving between two monitors with different content change).

To be tested