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/scrobbled_tracks_live/index_test.exs

@@ -56,7 +56,7 @@ defmodule MusicLibraryWeb.ScrobbledTracksLiveTest do
       session
       |> unwrap(fn view ->
         view
-        |> form("form[phx-submit='search']", %{query: "Unique Track"})
+        |> form("form[phx-submit='search']:not([phx-target])", %{query: "Unique Track"})
         |> render_submit()
       end)
       |> assert_has("p", "Unique Track Title")