refactor: Simpler project structure

[M123-dev]

What

Fixes: #3503
Followup from #3465

Before we had the dart entry points (void main) in /app/ together with the needed packages in the pubspec in /app/
This was unnecessary as the same can be achieved in the main code package /smooth_app/

Also this caused problems not only with pub on windows machines see #3503 and endless slack help requests but also problems with git bundled with an older win32 version which included max file length which was triggered by the nested code. (app/packages/smooth_app/packages/)

The main changes:

• moved the android, iOS, windows, ..., folder from /app/ to /smooth_app/
• Set one variable in smooth_app to null so that the assets again work
• Edited the entry points (the run button on the top now works again)
• Updated all workflows and scripts to work with this change

[monsieurtanuki]

Hi @M123-dev!
I cannot review 226 files and I don't understand fully the implications of what you're doing.
If you tested and if today the app smooth_app can be built correctly, that's good enough for me.

[M123-dev]

Local works without problems, but there is always a bit of luck involved with the automatic release. 🤞🏼