Fix: show success toast when saving column preferences

When saving column defaults, the flash was rendered inside the
turbo_stream.replace of the form element, placing it inside the
dropdown with incorrect styling instead of the top-level #flashes
container.

Fix: separate the flash into its own turbo_stream.append targeting
#flashes, matching the pattern used by destroy_product_variant and
clone turbo stream views.

Also tightened the spec to assert the flash appears in #flashes
with .flash.success class, not just that the text is present.

Fixes openfoodfoundation#13966

Author: gbathree

app/views/admin/column_preferences/bulk_update.turbo_stream.haml

@@ -1,3 +1,4 @@
 = turbo_stream.replace "bulk_admin_column_preferences_form" do
-  = render partial: "admin/shared/flashes", locals: { flashes: flash } if defined? flash
   = render partial: 'form', locals: { action: }
+= turbo_stream.append "flashes" do
+  = render partial: "admin/shared/flashes", locals: { flashes: flash }

spec/system/admin/products_v3/actions_spec.rb

@@ -85,9 +85,11 @@ def expect_other_columns_visible
       end
 
       def save_preferences
-        # Preference saved
         click_on "Save as default"
-        expect(page).to have_content "Column preferences saved"
+        # Flash appears at top level with success styling (not error), fixes #13966
+        within "#flashes" do
+          expect(page).to have_css ".flash.success", text: "Column preferences saved"
+        end
         refresh
       end
     end