rework email settings UX with guided flow and friendly errors

grouped providers by category, added per-provider key validation
with cross-provider detection, friendly delivery error messages,
retryable vs config error distinction, from-address in general
settings, and "Save settings" button to match admin conventions

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

test/berrypod_web/live/admin/settings_test.exs

@@ -240,6 +240,32 @@ defmodule BerrypodWeb.Admin.SettingsTest do
     end
   end
 
+  describe "from address" do
+    setup %{conn: conn, user: user} do
+      conn = log_in_user(conn, user)
+      %{conn: conn}
+    end
+
+    test "shows from address section", %{conn: conn} do
+      {:ok, _view, html} = live(conn, ~p"/admin/settings")
+
+      assert html =~ "From address"
+      assert html =~ "sender address"
+    end
+
+    test "saves from address", %{conn: conn} do
+      {:ok, view, _html} = live(conn, ~p"/admin/settings")
+
+      html =
+        view
+        |> form("form[phx-submit=\"save_from_address\"]", %{from_address: "shop@example.com"})
+        |> render_submit()
+
+      assert html =~ "From address saved"
+      assert Settings.get_setting("email_from_address") == "shop@example.com"
+    end
+  end
+
   describe "advanced section" do
     setup %{conn: conn, user: user} do
       conn = log_in_user(conn, user)