DEV: allow themes to render their own custom homepage (#26291)

This PR adds a theme modifier and route so that custom themes can opt to show their own homepage. See PR description for example usage.

lib/homepage_constraint.rb

@@ -9,7 +9,7 @@ class HomePageConstraint
     return @filter == "finish_installation" if SiteSetting.has_login_hint?
 
     current_user = CurrentUser.lookup_from_env(request.env)
-    homepage = current_user&.user_option&.homepage || SiteSetting.anonymous_homepage
+    homepage = current_user&.user_option&.homepage || HomepageHelper.resolve(request, current_user)
     homepage == @filter
   rescue Discourse::InvalidAccess, Discourse::ReadOnly
     false

lib/homepage_helper.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class HomepageHelper
+  def self.resolve(request = nil, current_user = nil)
+    return "custom" if ThemeModifierHelper.new(request: request).custom_homepage
+
+    current_user ? SiteSetting.homepage : SiteSetting.anonymous_homepage
+  end
+end