Fix OpenAPI spec for preferred_frontend endpoint

The spec was copied from another endpoint, including the operation id,
leading to scrubbing the valid parameters from the request and simply
not working.

lib/pleroma/web/api_spec/operations/frontend_settings_operation.ex

@@ -111,9 +111,9 @@ def available_frontends_operation() do
   def update_preferred_frontend_operation() do
     %Operation{
       tags: ["Frontends"],
-      summary: "Frontend Settings Profiles",
+      summary: "Update preferred frontend setting",
-      description: "List frontend setting profiles",
+      description: "Store preferred frontend in cookies",
-      operationId: "AkkomaAPI.FrontendSettingsController.available_frontends",
+      operationId: "AkkomaAPI.FrontendSettingsController.update_preferred_frontend",
       requestBody:
         request_body(
           "Frontend",
@@ -132,9 +132,11 @@ def update_preferred_frontend_operation() do
       responses: %{
         200 =>
           Operation.response("Frontends", "application/json", %Schema{
-            type: :array,
+            type: :object,
-            items: %Schema{
+            properties: %{
-              type: :string
+              frontend_name: %Schema{
+                type: :string
+              }
             }
           })
       }

test/pleroma/web/akkoma_api/frontend_settings_controller_test.exs

@@ -119,4 +119,18 @@ test "deletes a config" do
              ) == nil
     end
   end
+
+  describe "PUT /api/v1/akkoma/preferred_frontend" do
+    test "sets a cookie with selected frontend" do
+      %{conn: conn} = oauth_access(["read"])
+
+      response =
+        conn
+        |> put_req_header("content-type", "application/json")
+        |> put("/api/v1/akkoma/preferred_frontend", %{"frontend_name" => "pleroma-fe/stable"})
+
+      json_response_and_validate_schema(response, 200)
+      assert %{"preferred_frontend" => %{value: "pleroma-fe/stable"}} = response.resp_cookies
+    end
+  end
 end