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/settings_test.exs

@@ -113,7 +113,7 @@ defmodule BerrypodWeb.Admin.SettingsTest do
         })
         |> render_submit()
 
-      assert html =~ "Webhook signing secret saved"
+      assert has_element?(view, ".admin-inline-feedback-saved")
       assert html =~ "whsec_te•••456"
     end
 
@@ -261,7 +261,7 @@ defmodule BerrypodWeb.Admin.SettingsTest do
         |> form("form[phx-submit=\"save_from_address\"]", %{from_address: "shop@example.com"})
         |> render_submit()
 
-      assert html =~ "From address saved"
+      assert has_element?(view, ".admin-inline-feedback-saved")
       assert Settings.get_setting("email_from_address") == "shop@example.com"
     end
   end