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

src-tauri/src/commands_v2/settings.rs

@@ -246,7 +246,7 @@ pub fn v2_get_available_backends() -> Result<Vec<BackendInfo>, String> {
                 AudioBackendType::PipeWire => "PipeWire",
                 AudioBackendType::Alsa => "ALSA Direct",
                 AudioBackendType::Pulse => "PulseAudio",
-                AudioBackendType::SystemDefault => "System Audio",
+                AudioBackendType::SystemDefault => "System",
             };
 
             BackendInfo {