migrate admin forms to inline feedback

Replace put_flash calls with inline feedback for form saves:
- Email settings: "Now send a test email" after saving
- Settings: from address and signing secret saves
- Page editor: save button shows "Saved" checkmark

Inline feedback appears next to save buttons and auto-clears after
3 seconds. Banners (put_flash) remain for page-level outcomes like
deletions, state changes, and async operations.

Task 3 of notification overhaul. Theme editor skipped as it auto-saves.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/berrypod_web/live/admin/email_settings_test.exs

@@ -97,7 +97,7 @@ defmodule BerrypodWeb.Admin.EmailSettingsTest do
         })
         |> render_submit()
 
-      assert html =~ "Settings saved"
+      assert html =~ "Now send a test email"
       assert Settings.get_setting("email_adapter") == "brevo"
     end