[FEAT] App shell to display loading icon while daff.io is loading

[damienwebdev]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[x] Feature request
[ ] Documentation issue or request
[ ] Other... Please describe:

Current behavior

When the app is loading, there's a white screen

Expected behavior

Show loading icon while the app is loading.