Remove white flash on desktop

I ask claude for some other suggestions for things to add here, and
I agree with 2 of them:

- Remove a white flash by setting a default collor of the window as
  black.
- Explicitly set the maximized state.

gui/assets/main.scss

@@ -16,6 +16,7 @@ body {
 }
 
 #main {
+  visibility: visible;
   height: 100vh;
   display: flex;
   flex-direction: column;