FEATURE: Add thumbnails for chat image uploads (#24328)

Introduces the concept of image thumbnails in chat, prior to this we uploaded and used full size chat images within channels and direct messages.

The following changes are covered:
- Post processing of image uploads to create the thumbnail within Chat::MessageProcessor
- Extract responsive image ratios into CookedProcessorMixin (used for creating upload variations)
- Add thumbnail to upload serializer from plugin.rb
- Convert chat upload template to glimmer component using .gjs format
- Use thumbnail image within chat upload component (stores full size img in orig-src data attribute)
- Old uploads which don't have thumbnails will fallback to full size images in channels/DMs
- Update Magnific lightbox to use full size image when clicked
- Update Glimmer lightbox to use full size image (enables zooming for chat images)

plugins/chat/plugin.rb

@@ -253,6 +253,12 @@ after_initialize do
     object.chat_separate_sidebar_mode
   end
 
+  add_to_serializer(
+    :upload,
+    :thumbnail,
+    include_condition: -> { SiteSetting.chat_enabled && SiteSetting.create_thumbnails },
+  ) { object.thumbnail }
+
   RETENTION_SETTINGS_TO_USER_OPTION_FIELDS = {
     chat_channel_retention_days: :dismissed_channel_retention_reminder,
     chat_dm_retention_days: :dismissed_dm_retention_reminder,