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

test/pinchflat_web/api_auth_plug_test.exs

@@ -62,4 +62,4 @@ defmodule PinchflatWeb.ApiAuthPlugTest do
       assert %{status: 200} = conn
     end
   end
-end
+end