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.toml

@@ -20,6 +20,7 @@ drm = "0.12"
 drm-fourcc = "2"
 libc = "0.2"
 ashpd = { version = "0.13", features = ["tokio", "screencast"] }
+zbus = { version = "5", default-features = false, features = ["tokio"] }
 tokio = { version = "1", features = ["rt"] }
 pipewire = "0.9"
 libspa = "0.9"