redirect /admin/theme to on-site editor at /?edit=theme

- Phase 5 was already implemented (URL mode activation via ?edit param)
- Phase 6: Add RedirectController to redirect /admin/theme → /?edit=theme
- Update admin sidebar and dashboard links to point directly to /?edit=theme
- Delete old Admin.Theme.Index LiveView and template (no longer needed)
- Update tests for new redirect behavior

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

test/berrypod_web/live/admin/layout_test.exs

@@ -19,7 +19,7 @@ defmodule BerrypodWeb.Admin.LayoutTest do
 
       assert has_element?(view, ~s(a[href="/admin"]), "Dashboard")
       assert has_element?(view, ~s(a[href="/admin/orders"]), "Orders")
-      assert has_element?(view, ~s(a[href="/admin/theme"]), "Theme")
+      assert has_element?(view, ~s(a[href="/?edit=theme"]), "Theme")
       assert has_element?(view, ~s(a[href="/admin/settings"]), "Settings")
     end
 
@@ -45,24 +45,6 @@ defmodule BerrypodWeb.Admin.LayoutTest do
     end
   end
 
-  describe "theme editor layout" do
-    setup %{conn: conn, user: user} do
-      %{conn: log_in_user(conn, user)}
-    end
-
-    test "does not render sidebar", %{conn: conn} do
-      {:ok, _view, html} = live(conn, ~p"/admin/theme")
-
-      refute html =~ "admin-drawer"
-    end
-
-    test "shows back link to admin", %{conn: conn} do
-      {:ok, view, _html} = live(conn, ~p"/admin/theme")
-
-      assert has_element?(view, ~s(a[href="/admin"]), "Admin")
-    end
-  end
-
   describe "admin bar on shop pages" do
     setup do
       {:ok, _} = Berrypod.Settings.set_site_live(true)