DEV: Extract theme resolution to a helper (#27426)

This ensures that the theme id is resolved as early as possible in the
request cycle. This is necessary for the custom homepage to skip
preloading the wrong data.

lib/theme_resolver.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module ThemeResolver
+  def self.resolve_theme_id(request, guardian, current_user)
+    return request.env[:resolved_theme_id] if request.env[:resolved_theme_id] != nil
+
+    theme_id = nil
+
+    if (preview_theme_id = request[:preview_theme_id]&.to_i) &&
+         guardian.allow_themes?([preview_theme_id], include_preview: true)
+      theme_id = preview_theme_id
+    end
+
+    user_option = current_user&.user_option
+
+    if theme_id.blank? && request.cookie_jar[:theme_ids].present?
+      ids, seq = request.cookie_jar[:theme_ids]&.split("|")
+      id = ids&.split(",")&.map(&:to_i)&.first
+      if id.present? && seq && seq.to_i == user_option&.theme_key_seq.to_i
+        theme_id = id if guardian.allow_themes?([id])
+      end
+    end
+
+    if theme_id.blank?
+      ids = user_option&.theme_ids || []
+      theme_id = ids.first if guardian.allow_themes?(ids)
+    end
+
+    if theme_id.blank? && SiteSetting.default_theme_id != -1 &&
+         guardian.allow_themes?([SiteSetting.default_theme_id])
+      theme_id = SiteSetting.default_theme_id
+    end
+
+    request.env[:resolved_theme_id] = theme_id
+  end
+end