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.

src/backend_detect.rs

@@ -37,11 +37,10 @@ impl Dispatch<WlRegistry, GlobalListContents> for RegistryLs {
     }
 }
 
-// 通过 D-Bus 检测 XDG Desktop Portal 的 ScreenCast 接口是否可用
-// 尝试创建 Screencast proxy，如果 Portal 服务未运行则返回 false
+// CAUTION: must NOT use ashpd here — ashpd caches zbus::Connection in a global
+// OnceLock; if the tokio runtime owning that connection is dropped before
+// setup_portal() runs, the cached connection becomes dead and hangs forever.
 fn check_portal_available() -> bool {
-    use ashpd::desktop::screencast::Screencast;
-
     let rt = match tokio::runtime::Runtime::new() {
         Ok(rt) => rt,
         Err(e) => {
@@ -51,30 +50,43 @@ fn check_portal_available() -> bool {
     };
 
     rt.block_on(async {
-        let proxy = match Screencast::new().await {
-            Ok(p) => p,
+        let conn = match zbus::Connection::session().await {
+            Ok(c) => c,
             Err(e) => {
-                tracing::info!("Portal not available: {e}");
+                tracing::info!("D-Bus session bus unavailable: {e}");
                 return false;
             }
         };
 
-        // Verify the portal actually exposes ScreenCast capabilities,
-        // not just that the D-Bus service is running.
-        match proxy.available_source_types().await {
-            Ok(types) if !types.is_empty() => {
-                tracing::info!("Portal ScreenCast available (source types: {types:?})");
+        let inner: zbus::Proxy = match zbus::proxy::Builder::new(&conn)
+            .destination("org.freedesktop.portal.Desktop")
+            .and_then(|b| b.path("/org/freedesktop/portal/desktop"))
+            .and_then(|b| b.interface("org.freedesktop.portal.ScreenCast"))
+        {
+            Ok(b) => match b.build().await {
+                Ok(p) => p,
+                Err(e) => {
+                    tracing::info!("Portal ScreenCast interface not available: {e}");
+                    return false;
+                }
+            },
+            Err(e) => {
+                tracing::info!("Portal ScreenCast proxy build failed: {e}");
+                return false;
+            }
+        };
+
+        let version = match inner.get_property::<u32>("version").await {
+            Ok(version) => {
+                tracing::info!("Portal ScreenCast available (version: {version})");
                 true
             }
-            Ok(types) => {
-                tracing::info!("Portal ScreenCast proxy exists but no source types available ({types:?})");
-                false
-            }
             Err(e) => {
-                tracing::info!("Portal ScreenCast available_source_types query failed: {e}");
+                tracing::info!("Portal ScreenCast version query failed: {e}");
                 false
             }
-        }
+        };
+        version
     })
 }