fix(audio): detect PipeWire via runtime socket so the backend is not hidden without pactl (#466)

is_pipewire_available() shelled out to `pactl info`, which lives in
pulseaudio-utils and is absent on PipeWire-only systems — hiding the PipeWire
backend even though PipeWire was running. Detect via the
$XDG_RUNTIME_DIR/pipewire-0 socket first (no extra package required), with
pw-cli and pactl as fallbacks.

cc

crates/qbz-audio/src/backend.rs

@@ -350,9 +350,26 @@ impl BackendManager {
 
     #[cfg(target_os = "linux")]
     fn is_pipewire_available() -> bool {
-        // Check if PipeWire/PulseAudio is available using pactl
-        // PipeWire provides PulseAudio compatibility, so pactl works for both
-        // This is more reliable than pw-cli, especially in sandboxed environments (Flatpak)
+        // Detect PipeWire via its runtime socket ($XDG_RUNTIME_DIR/pipewire-0),
+        // which exists whenever PipeWire is running. Unlike `pactl`, this does
+        // NOT require pulseaudio-utils to be installed — PipeWire-only systems
+        // frequently lack it, which used to hide the PipeWire backend entirely
+        // (issue #466). pw-cli / pactl remain as fallbacks for unusual setups
+        // (e.g. a non-default socket name, or Flatpak where the socket path
+        // differs but the pulse shim is bridged).
+        if let Some(runtime_dir) = std::env::var_os("XDG_RUNTIME_DIR") {
+            if std::path::Path::new(&runtime_dir).join("pipewire-0").exists() {
+                return true;
+            }
+        }
+        if std::process::Command::new("pw-cli")
+            .args(["info", "0"])
+            .output()
+            .map(|o| o.status.success())
+            .unwrap_or(false)
+        {
+            return true;
+        }
         std::process::Command::new("pactl")
             .arg("info")
             .output()