Adopt bread_utils::gtk_popup for the launcher overlay

Replace the hand-rolled layer-shell window, visible-row Up/Down
navigation, and click-outside-to-close gesture with the shared
helpers already used by breadclip. Keep singleton::toggle_or_kill
and bind_window_auto.

Cargo.lock

@@ -121,6 +121,8 @@ source = "git+https://git.breadway.dev/Breadway/bread-ecosystem?tag=v0.7.2#30517
 dependencies = [
  "bread-shared",
  "dirs",
+ "gtk4",
+ "gtk4-layer-shell",
  "serde",
  "serde_json",
 ]