add admin email settings page with provider selection

Card radio component for picking email providers (SMTP, SendGrid, Mailjet, etc.)
with instant client-side switching via JS hook. Adapter configs are pre-rendered
and toggled without a server round-trip. Secrets are preserved when re-saving
with blank password fields. Includes from address field, test email sending,
and disconnect flow.

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

test/berrypod_web/live/setup/onboarding_test.exs

@@ -103,8 +103,8 @@ defmodule BerrypodWeb.Setup.OnboardingTest do
 
       html =
         view
-        |> element(~s(button[phx-value-type="printify"]))
-        |> render_click()
+        |> form(~s(form[phx-change="select_provider"]), %{provider_select: %{type: "printify"}})
+        |> render_change()
 
       assert html =~ "API token"
       assert html =~ "Printify"