Merge upstream/css-cleanup into master

Resolves conflicts between upstream/master and upstream/css-cleanup:
- CSS: take css-cleanup versions (button/link style unification, comment cleanup,
  table .button styles, [name=cancel]/.auxiliary styles)
- application_helper.rb: use SVG icon for flash message close button (css-cleanup)
- users_test.rb: use single-quote style (css-cleanup), keep sole-admin test (master)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/locales/en.yml

@@ -156,7 +156,7 @@ en:
       edit:
         password_html: 'New password:%{password_length_hint_html}'
         update_password: Update password
-    registrations:
+    profiles:
       new:
         password_html: 'Password:%{password_length_hint_html}'
         password_confirmation: 'Retype password:'
@@ -195,7 +195,6 @@ en:
   cancel: Cancel
   delete: Delete
   :no: 'no'
-  or: or
   register: Register
   sign_in: Sign in
   recover_password: Recover password