Add missing Account/Delete confirmation page

The POST Delete(UnregisterViewModel) action and the GET Delete() action
both existed in AccountController, but Views/Account/Delete.cshtml was
missing — so GET /Account/Delete had no page to render. Add the
confirmation view (posts back to Delete with the user's UserId, with a
"Confirmer la suppression" button and an "Annuler" cancel link).

Add a smoke test: an authenticated GET /Account/Delete returns 200 and
the confirmation page body.

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

src/Yavsc.Org/Controllers/Accounting/AccountController.cs

@@ -1061,6 +1061,7 @@ IHtmlLocalizerFactory htmlLocalizerFactory,
             }
             return View();
         }
+
         [HttpPost, Authorize]
         public async Task<IActionResult> Delete(UnregisterViewModel model)
         {