share provider connection logic between setup wizard and providers form

Extract Products.connect_provider/2 that tests the connection, fetches
shop_id, creates the record, and enqueues sync. Both the setup wizard
and the providers form now use this shared function instead of
duplicating the flow. Also makes the products empty state context-aware
(distinguishes "no provider" from "provider connected but no products").

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/berrypod_web/live/admin/providers_test.exs

@@ -136,6 +136,12 @@ defmodule BerrypodWeb.Admin.ProvidersTest do
 
   describe "form - new" do
     setup %{conn: conn, user: user} do
+      Application.put_env(:berrypod, :provider_modules, %{
+        "printify" => MockProvider
+      })
+
+      on_exit(fn -> Application.delete_env(:berrypod, :provider_modules) end)
+
       %{conn: log_in_user(conn, user)}
     end
 
@@ -170,6 +176,10 @@ defmodule BerrypodWeb.Admin.ProvidersTest do
     end
 
     test "saves new connection", %{conn: conn} do
+      expect(MockProvider, :test_connection, fn _conn ->
+        {:ok, %{shop_name: "My Printify Shop", shop_id: 12345}}
+      end)
+
       {:ok, view, _html} = live(conn, ~p"/admin/providers/new")
 
       {:ok, _view, html} =