FIX: Preload fonts before rerendering wizard style canvas (#22361)

]When changing fonts in the `/wizard/steps/styling` step of
the wizard, users would not see the font loaded straight away,
having to switch to another one then back to the original to
see the result. This is because we are using canvas to render
the style preview and this fails with a Chrome-based intervention
when font loading is taking too long:

> [Intervention] Slow network is detected. See
https://www.chromestatus.com/feature/5636954674692096 for more details.
Fallback font will be used while loading:
https://sea2.discourse-cdn.com/business7/fonts/Roboto-Bold.ttf?v=0.0.9

We can get around this by manually loading the fonts selected using
the FontFace JS API when the user selects them and before rerendering
the canvas. This just requires preloading more information about the
fonts if the user is admin so the wizard can query this data.

spec/requests/application_controller_spec.rb

@@ -1133,4 +1133,96 @@ RSpec.describe ApplicationController do
       end
     end
   end
+
+  describe "preloading data" do
+    def preloaded_json
+      JSON.parse(
+        Nokogiri::HTML5.fragment(response.body).css("div#data-preloaded").first["data-preloaded"],
+      )
+    end
+
+    context "when user is anon" do
+      it "preloads the relevant JSON data" do
+        get "/latest"
+        expect(response.status).to eq(200)
+        expect(preloaded_json.keys).to match_array(
+          [
+            "site",
+            "siteSettings",
+            "customHTML",
+            "banner",
+            "customEmoji",
+            "isReadOnly",
+            "isStaffWritesOnly",
+            "activatedThemes",
+            "#{TopicList.new("latest", Fabricate(:anonymous), []).preload_key}",
+          ],
+        )
+      end
+    end
+
+    context "when user is regular user" do
+      fab!(:user) { Fabricate(:user) }
+
+      before { sign_in(user) }
+
+      it "preloads the relevant JSON data" do
+        get "/latest"
+        expect(response.status).to eq(200)
+        expect(preloaded_json.keys).to match_array(
+          [
+            "site",
+            "siteSettings",
+            "customHTML",
+            "banner",
+            "customEmoji",
+            "isReadOnly",
+            "isStaffWritesOnly",
+            "activatedThemes",
+            "#{TopicList.new("latest", Fabricate(:anonymous), []).preload_key}",
+            "currentUser",
+            "topicTrackingStates",
+            "topicTrackingStateMeta",
+          ],
+        )
+      end
+    end
+
+    context "when user is admin" do
+      fab!(:user) { Fabricate(:admin) }
+
+      before { sign_in(user) }
+
+      it "preloads the relevant JSON data" do
+        get "/latest"
+        expect(response.status).to eq(200)
+        expect(preloaded_json.keys).to match_array(
+          [
+            "site",
+            "siteSettings",
+            "customHTML",
+            "banner",
+            "customEmoji",
+            "isReadOnly",
+            "isStaffWritesOnly",
+            "activatedThemes",
+            "#{TopicList.new("latest", Fabricate(:anonymous), []).preload_key}",
+            "currentUser",
+            "topicTrackingStates",
+            "topicTrackingStateMeta",
+            "fontMap",
+          ],
+        )
+      end
+
+      it "generates a fontMap" do
+        get "/latest"
+        expect(response.status).to eq(200)
+        font_map = JSON.parse(preloaded_json["fontMap"])
+        expect(font_map.keys).to match_array(
+          DiscourseFonts.fonts.filter { |f| f[:variants].present? }.map { |f| f[:key] },
+        )
+      end
+    end
+  end
 end