[PR] Flutter loading time and web renderers

[LuchoTurtle]

related to dwyl/app#315

This PR improves the Flutter app by adding a splash screen and writes a section about the Lighthouse metrics difference between using a canvaskit and html renderer.

[nelsonic]

When I visit: https://flutter-phoenix-channels-frontend.fly.dev/ I see the loading screen:

When I manually refresh the page it loads the actual app:
https://user-images.githubusercontent.com/194400/221338985-17058d41-feee-4cf1-9e9b-3451e035e7da.mov

I wasn't able to reproduce this in any other browser so I consider it a "fluke" not an issue worth reporting. 👌

[nelsonic]

@LuchoTurtle good write-up. You've changed my mind on the splash screens. It's not as bad as I suspected and substantially improves perf score on lighthouse. Nice one. 👌

[nelsonic]

CC: @SimonLab worth checking this one over to update your knowledge. 💭

[LuchoTurtle]

When I manually refresh the page it loads the actual app

This is perhaps because the initialization of the engine only occurs when the user first visits the page. The downloaded elements are cached on subsequent visits.