refactor(dossier): cleanup user controller

[tchak]

Ce nettoyage est fait pour préparer le passage vers l'édition par streams :

• last_champ_updated_at est touché seulement sur les brouillons
• On passe explicitement @dossier_for_editing aux views. Ça permet de driver l'usage depuis le controller sans logique dans les views. Cette variable disparaîtra après le passage complet aux streams
• Supprimer du code mort

[codecov]

Codecov Report

Attention: Patch coverage is 96.10390% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.38%. Comparing base (7932085) to head (1f5dd3b).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
app/controllers/users/dossiers_controller.rb	92.59%	2 Missing ⚠️
app/models/concerns/dossier_state_concern.rb	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11067      +/-   ##
==========================================
+ Coverage   84.35%   84.38%   +0.03%     
==========================================
  Files        1176     1176              
  Lines       25944    25950       +6     
  Branches     4898     4903       +5     
==========================================
+ Hits        21886    21899      +13     
+ Misses       4058     4051       -7     

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[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ ActionView::Template::Error: virus_scanner_error? delegated to blob, but blob is nil (ActionView::Template::Error) Champs::PieceJustificativeController#update View Issue
• ‼️ **ActionView::Template::Error: undefined method include?' for false (ActionView::Template::Error)** Users::DossiersController#modifier` View Issue
• ‼️ **ActionView::Template::Error: undefined method push' for nil (ActionView::Template::Error)** Administrateurs::ProceduresController#apercu` View Issue
• ‼️ **ActionView::Template::Error: undefined method include?' for false (ActionView::Template::Error)** Users::DossiersController#modifier` View Issue

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