Normalize SDL3 HiDPI window scaling across platforms

[Copilot]

SDL3 windows on Windows HiDPI displays used a fixed 1.0 scale, making fonts and widgets undersized. Resize handling also conflated logical window dimensions with physical framebuffer dimensions.

• Per-window DPI

  • Use SDL_GetWindowDisplayScale() for high-density windows on all platforms.
  • Refresh scaling when windows move between displays.

• Resize semantics

  • Apply SDL_EVENT_WINDOW_RESIZED to logical component bounds.
  • Treat SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED as a framebuffer repaint/renderer update only.

• Display bookkeeping

  • Rebuild the screen list on display updates instead of accumulating duplicates.

const auto scale = SDL_GetWindowDisplayScale (window);
return scale > 0.0f ? scale : 1.0f;