Add proper HighDPI support across all platforms

[emoon]

Summary

Add proper HighDPI support across all platforms (macOS, Windows, Linux) so the UI renders crisply on modern high-resolution displays.

Current Behavior

• macOS: Retina support explicitly disabled (setWantsBestResolutionOpenGLSurface:NO)
• Windows: No DPI awareness manifest, causing Windows to bitmap-scale the entire application (blurry menus and UI)
• Linux: SDL2 window created without SDL_WINDOW_ALLOW_HIGHDPI flag

Proposed Solution

• Add DPI awareness manifest for Windows
• Enable Retina support on macOS
• Add SDL_WINDOW_ALLOW_HIGHDPI flag on Linux
• Add scale factor management to EMGUI
• Separate OpenGL viewport (physical pixels) from projection (logical pixels)
• Support TTF font re-rasterization at different scales
• Handle mouse coordinate transformation
• Add user-configurable UI scale option (100%, 125%, 150%, 200%)

References

• Research document: research/highdpi-scaling-support.md