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>

lib/berrypod_web/controllers/redirect_controller.ex

@@ -0,0 +1,10 @@
+defmodule BerrypodWeb.RedirectController do
+  use BerrypodWeb, :controller
+
+  @doc """
+  Redirects /admin/theme to the on-site theme editor.
+  """
+  def theme(conn, _params) do
+    redirect(conn, to: "/?edit=theme")
+  end
+end