Add admin/editeur/user roles for the associates

root (renamed from the default admin/admin account) keeps full,
unrestricted access outside any role system. These three cover the
actual associates:

- user: read-only, 'access content' only -- can browse /lignes and
  /dashboard, nothing else.
- editeur: 'access content' + create/edit any/delete any
  ligne_comptable content -- can enter and correct accounting lines,
  no site configuration.
- admin: identical permission set to editeur for now (per instruction,
  actual config permissions to be scoped later).

Left the pre-existing "administrator" (is_admin bypass -- too broad)
and "content_editor" (generic Standard-recipe scaffolding, permissions
unrelated to ligne_comptable) roles untouched but unused; worth
pruning later if nothing ends up using them.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

config/sync/user.role.user.yml

@@ -0,0 +1,12 @@
+uuid: 24f4e46d-c714-486d-b000-f092eb727470
+langcode: en
+status: true
+dependencies:
+  module:
+    - system
+id: user
+label: 'Utilisateur (lecture seule)'
+weight: 4
+is_admin: false
+permissions:
+  - 'access content'