rework email settings for true progressive enhancement

Render all adapter field sections in the form with CSS :has(:checked)
controlling visibility. Selecting a provider instantly shows its config
fields — no JS, no page reload, no server round-trip needed.

- Render all 6 adapter configs with data-adapter attribute
- CSS :has(:checked) show/hide rules per adapter in admin stylesheet
- Namespace field names per adapter (email[brevo][api_key] etc)
- Drop 4 transactional-only providers (Resend, Postmark, Mailgun, MailPace)
- Remove noscript "Switch provider" button and controller redirect workaround
- Remove configured_adapter hidden input tracking
- Hide JS-only test email button for no-JS users via noscript style
- LiveView progressively enhances with async save and test email

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

lib/berrypod_web/controllers/email_settings_controller.ex

@@ -10,27 +10,22 @@ defmodule BerrypodWeb.EmailSettingsController do
   alias Berrypod.Mailer
 
   def update(conn, %{"email" => params}) do
-    selected = params["adapter"]
-    configured = params["configured_adapter"]
+    adapter_key = params["adapter"]
+    # Fields are namespaced: email[brevo][api_key] → params["brevo"]["api_key"]
+    adapter_params = params[adapter_key] || %{}
 
-    if selected != configured do
-      # User changed adapter radio but config fields are for the old adapter.
-      # Redirect to show the new adapter's config fields.
-      redirect(conn, to: ~p"/admin/settings/email?adapter=#{selected}")
-    else
-      case Mailer.save_config(selected, params, conn.assigns.current_scope.user.email) do
-        {:ok, _adapter_info} ->
-          conn
-          |> put_flash(:info, "Settings saved — send a test email to check it works")
-          |> redirect(to: ~p"/admin/settings/email")
+    case Mailer.save_config(adapter_key, adapter_params, conn.assigns.current_scope.user.email) do
+      {:ok, _adapter_info} ->
+        conn
+        |> put_flash(:info, "Settings saved — send a test email to check it works")
+        |> redirect(to: ~p"/admin/settings/email")
 
-        {:error, field_errors} when is_map(field_errors) ->
-          message = field_errors |> Map.values() |> Enum.join(". ")
+      {:error, field_errors} when is_map(field_errors) ->
+        message = field_errors |> Map.values() |> Enum.join(". ")
 
-          conn
-          |> put_flash(:error, message)
-          |> redirect(to: ~p"/admin/settings/email?adapter=#{selected}")
-      end
+        conn
+        |> put_flash(:error, message)
+        |> redirect(to: ~p"/admin/settings/email")
     end
   end