Open universal search modal instantly via client-side JS

Replace phx-click server event with Fluxon.open_dialog for zero-latency
modal opening. The modal DOM is now always present (no :if conditional) so
it can be shown instantly; the server syncs state in the background.

Fix test selectors that now match the always-present modal form by scoping
to forms without phx-target (which the modal form always has).

test/music_library_web/live/universal_search_live/index_test.exs

@@ -23,7 +23,7 @@ defmodule MusicLibraryWeb.UniversalSearchLive.IndexTest do
     test "modal is hidden by default", %{conn: conn} do
       conn
       |> visit(~p"/collection")
-      |> refute_has("#universal-search-root")
+      |> assert_has("#universal-search-root[hidden]")
     end
 
     test "modal opens when search button is clicked", %{conn: conn} do