feat: Bring the "Hot reload" feature back

[g123k]

Hi everyone!

After the migration to GoRouter, the Hot Reload feature from Dart/Flutter didn't work anymore and was a real pain.
The problem was coming from the GoRouter object being recreated on every Hot Reload.

By using a singleton instead, we're now safe and the Hot Reload works again ☺️

[codecov-commenter]

Codecov Report

Merging #4279 (02a5fb4) into develop (505c0d1) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop    #4279      +/-   ##
===========================================
- Coverage    10.87%   10.87%   -0.01%     
===========================================
  Files          285      285              
  Lines        14172    14175       +3     
===========================================
  Hits          1541     1541              
- Misses       12631    12634       +3     

Impacted Files	Coverage Δ
.../smooth_app/lib/pages/navigator/app_navigator.dart	0.00% <0.00%> (ø)

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[g123k]

Thanks for your review, let's merge this one!