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>

test/berrypod_web/controllers/email_settings_controller_test.exs

@@ -24,8 +24,7 @@ defmodule BerrypodWeb.EmailSettingsControllerTest do
         post(conn, ~p"/admin/settings/email", %{
           email: %{
             adapter: "brevo",
-            configured_adapter: "brevo",
-            api_key: "xkeysib-abc123def456"
+            brevo: %{api_key: "xkeysib-abc123def456"}
           }
         })
 
@@ -34,19 +33,10 @@ defmodule BerrypodWeb.EmailSettingsControllerTest do
       assert Settings.get_setting("email_adapter") == "brevo"
     end
 
-    test "redirects with adapter param when adapter changed", %{conn: conn} do
-      conn =
-        post(conn, ~p"/admin/settings/email", %{
-          email: %{adapter: "resend", configured_adapter: "brevo", api_key: ""}
-        })
-
-      assert redirected_to(conn) == ~p"/admin/settings/email?adapter=resend"
-    end
-
     test "redirects with error on validation failure", %{conn: conn} do
       conn =
         post(conn, ~p"/admin/settings/email", %{
-          email: %{adapter: "brevo", configured_adapter: "brevo", api_key: ""}
+          email: %{adapter: "brevo", brevo: %{api_key: ""}}
         })
 
       assert redirected_to(conn) =~ ~p"/admin/settings/email"
@@ -56,8 +46,8 @@ defmodule BerrypodWeb.EmailSettingsControllerTest do
 
   describe "POST /admin/settings/email/test" do
     test "sends test email and redirects", %{conn: conn} do
-      Settings.put_setting("email_adapter", "postmark")
-      Settings.put_secret("email_postmark_api_key", "pm_test_abc")
+      Settings.put_setting("email_adapter", "brevo")
+      Settings.put_secret("email_brevo_api_key", "xkeysib-test-abc")
 
       conn = post(conn, ~p"/admin/settings/email/test")