DEV: Refactor subclasses in ThemeSettingsManager to individual files (#25605)

Why this change?

One Ruby class per file improves readability

lib/theme_settings_manager/upload.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+class ThemeSettingsManager::Upload < ThemeSettingsManager
+  def value
+    has_record? ? cdn_url(db_record.value) : default
+  end
+
+  def default
+    upload_id = default_upload_id
+    return if upload_id.blank?
+
+    cdn_url(upload_id)
+  end
+
+  def value=(new_value)
+    if new_value.present?
+      if new_value == default
+        new_value = default_upload_id
+      else
+        upload = ::Upload.find_by(url: new_value)
+        new_value = upload.id if upload.present?
+      end
+    end
+
+    super(new_value)
+  end
+
+  private
+
+  def cdn_url(upload_id)
+    return if upload_id.blank?
+
+    upload = ::Upload.find_by_id(upload_id.to_i)
+    return if upload.blank?
+
+    Discourse.store.cdn_url(upload.url)
+  end
+
+  def default_upload_id
+    theme_field =
+      theme.theme_fields.find_by(name: @default, type_id: ThemeField.types[:theme_upload_var])
+    return if theme_field.blank?
+
+    theme_field.upload_id
+  end
+end