fix(audio): default to shared System output and resolve Auto to a concrete backend (#470)

OOTB playback froze (seekbar advancing, no audio) on hosts where the
configured backend was not usable — e.g. PipeWire as the Linux default but
`pactl` absent (minimal Debian/Ubuntu). "Auto" (a null backend_type) fell
through to a legacy CPAL path that forced the track rate onto the shared
default device, producing a suspended/silent stream and leaving a
process-wide stuck audio handle that survived Reset.

- Add a cross-platform "System" backend (CpalDefaultBackend now on Linux
  too): opens the OS default output at its negotiated rate with rodio
  resampling and an explicit ~100ms CPAL buffer (MixerDeviceSink has zero
  internal buffering, so the bare default underran and stayed suspended).
  Shared, no exclusivity, no `pactl` — mixes with other apps like any
  normal player.
- Make System the OOTB default and what Reset lands on; seed it on first
  run. Drop the per-startup #375 NULL->PipeWire backfill migration.
- Resolve a null backend_type to System in the player so the legacy CPAL
  path is never reached (covers legacy installs and headless callers).
- "Auto" in Settings is now a resolve-and-set action: it picks the best
  available backend (PipeWire if available, else System), stores that
  concrete value and updates the dropdown + device picker, so backend_type
  is never null.
- Explicit PipeWire (DAC passthrough) and ALSA Direct (exclusive) configs
  are untouched and remain bit-perfect.

cc

crates/qbz-player/src/player/mod.rs

@@ -759,16 +759,16 @@ fn try_init_stream_with_backend(
     channels: u16,
     state: &SharedState,
 ) -> Option<Result<StreamType, String>> {
-    // Check if backend system is configured.
-    // On macOS, default to SystemDefault when not explicitly set,
-    // so CoreAudio device probing and sample rate switching are active.
-    let backend_type = audio_settings.backend_type.or_else(|| {
-        if cfg!(target_os = "macos") {
-            Some(qbz_audio::AudioBackendType::SystemDefault)
-        } else {
-            None
-        }
-    })?;
+    // A None backend_type means "Auto" / unset. Resolve it to SystemDefault on
+    // every platform instead of returning None — returning None made the caller
+    // fall through to the legacy CPAL path, which forced the track rate onto the
+    // shared default device: that froze the seekbar with no audio AND left a
+    // process-wide stuck audio handle that survived Reset (#470). "Auto" is
+    // resolved to a concrete backend in the UI; this is the backend-side safety
+    // net for any remaining None (legacy installs, headless callers).
+    let backend_type = audio_settings
+        .backend_type
+        .unwrap_or(qbz_audio::AudioBackendType::SystemDefault);
 
     log::info!(
         "Using backend system: {:?} (device: {:?}, plugin: {:?})",