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/api_auth_plug.ex

@@ -12,7 +12,6 @@ defmodule PinchflatWeb.ApiAuthPlug do
   """
 
   import Plug.Conn
-  alias Pinchflat.Settings
 
   def init(opts), do: opts
 
@@ -47,4 +46,4 @@ defmodule PinchflatWeb.ApiAuthPlug do
     |> send_resp(401, ~s({"errors":{"detail":"Unauthorized"}}))
     |> halt()
   end
-end
+end