FIX: N+1 in admin themes page (#32763)

`strict_loading` was added to prevent it happening in the future. Few
adjustments had to be made:
- include color_scheme and color_scheme_colors, also for parent and
child themes;
- internal translations were using preload_fields, but it was too deep
to correctly use preloaded tables. I had to pass
`preloaded_locale_fields` manually;
- include theme in color_scheme.

Before:
<img width="663" alt="Screenshot 2025-05-15 at 3 43 47 pm"
src="https://github.com/user-attachments/assets/b55ce11e-80cb-43eb-8e31-940b0e9859f3"
/>

After:
<img width="665" alt="Screenshot 2025-05-16 at 11 29 00 am"
src="https://github.com/user-attachments/assets/f00bac19-f64b-4048-b220-4d0a9d90a929"
/>

app/controllers/admin/themes_controller.rb

@@ -169,8 +169,10 @@ class Admin::ThemesController < Admin::AdminController
   end
 
   def index
-    @themes = Theme.include_relations.order(:name)
-    @color_schemes = ColorScheme.all.includes(:theme, color_scheme_colors: :color_scheme).to_a
+    @themes = Theme.strict_loading.include_relations.order(:name)
+
+    @color_schemes =
+      ColorScheme.strict_loading.all.includes(:theme, color_scheme_colors: :color_scheme).to_a
 
     payload = {
       themes: serialize_data(@themes, ThemeSerializer),