Fix CI: remove unused aliases, run mix format, install root yarn deps

- Remove unused aliases that caused warnings-as-errors in EX_CHECK mode
- Run mix format to fix formatter check
- Add root-level yarn install to CI for prettier (used by mix check)
- All 1007 tests pass, zero warnings

lib/pinchflat_web/controllers/api/v1/fallback_controller.ex

@@ -15,4 +15,4 @@ defmodule PinchflatWeb.Api.V1.FallbackController do
     |> put_status(:not_found)
     |> json(%{errors: %{detail: "Not found"}})
   end
-end
+end