fix: 4348 - simplified app navigator code

[monsieurtanuki]

What

• While reviewing fix: food preferences redirecting to a 404 page #4348, I noticed that we used enum in a way that would cause bugs (one fixed by fix: food preferences redirecting to a 404 page #4348, another one left alone).
• I simplified the code: here it's not really OOP but it's easy to understand and to maintain (and I don't think OOP would really be appropriate here anyway).

Part of

• fix: food preferences redirecting to a 404 page #4348

[codecov-commenter]

Codecov Report

Merging #4355 (da4f4c6) into develop (580b822) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff            @@
##           develop    #4355   +/-   ##
========================================
  Coverage    10.45%   10.46%           
========================================
  Files          291      291           
  Lines        14409    14404    -5     
========================================
  Hits          1507     1507           
+ Misses       12902    12897    -5     

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

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[github-actions]

You can test this PR on: https://app.waldo.com/applications/app-19d476740ba1bb36/sessions