perf(ui): move search IPC off the GTK main thread

Search queries in daemon mode now run on a background thread via
DaemonHandle::query_async(). Results are posted back to the main
thread via glib::spawn_future_local + futures_channel::oneshot.
The GTK event loop is never blocked by IPC, eliminating perceived
input lag.

Local mode (dmenu) continues to use synchronous search since it
has no IPC overhead.

Cargo.lock

@@ -2542,6 +2542,7 @@ dependencies = [
  "clap",
  "dirs",
  "env_logger",
+ "futures-channel",
  "glib-build-tools",
  "gtk4",
  "gtk4-layer-shell",