Fix wasm compilation/rendering

[Fabi019]

Thank you for this great UI library! While trying it out in a WASM environment I found and fixed some small issues:

• Compilation errors due to some non wasm compatible types used (muda::Menu, std::time::Instant)
• No render output due to self.paint_state not getting set to Initialized without paint called
• Invisible text on label/buttons in the webgpu example -> Manually set the font_family to "Fira Sans"