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/scrobble_rules_live/index_test.exs

@@ -140,7 +140,7 @@ defmodule MusicLibraryWeb.ScrobbleRulesLiveTest do
       {:ok, index_live, _html} = live(conn, ~p"/scrobble-rules")
 
       index_live
-      |> form("form[phx-change='search']", query: scrobble_rule.match_value)
+      |> form("form[phx-change='search']:not([phx-target])", query: scrobble_rule.match_value)
       |> render_change()
 
       assert_patch(index_live)