fix(backend_detect): use raw zbus for portal check to avoid OnceLock connection poisoning

ashpd caches zbus::Connection in a global OnceLock. When check_portal_available()
created a Screencast proxy, the connection was cached there. When the function
returned and its tokio Runtime dropped, the cached connection became dead.
Subsequent setup_portal() calls reused this dead connection and hung forever.

Fix: replace ashpd Screencast proxy with direct zbus D-Bus interface check,
which does not touch the ashpd global connection cache.

Add examples/test_portal.rs for minimal Portal ScreenCast testing.

Cargo.lock

@@ -1788,6 +1788,7 @@ dependencies = [
  "wayland-client",
  "wayland-protocols",
  "wayland-protocols-wlr",
+ "zbus",
 ]
 
 [[package]]