fix: revert BufferSize::Fixed for PipeWire, use BufferSize::Default

PipeWire manages its own buffer scheduling via quantum. Forcing a fixed
buffer size (sample_rate / 10 = 19200 frames at 192kHz) conflicts with
PipeWire's internal scheduling and causes buffer underruns that produce
audible noise.

BufferSize::Fixed is kept only for:
- Exclusive mode (512 frames, low latency)
- ALSA backend (direct ALSA, not through PipeWire)

crates/qbz-audio/src/pipewire_backend.rs

@@ -252,8 +252,10 @@ impl AudioBackend for PipeWireBackend {
             buffer_size: if config.exclusive_mode {
                 BufferSize::Fixed(512)  // Lower latency for exclusive mode
             } else {
-                // ~100ms period for fewer CPU wakeups (matches previous vendored cpal tuning)
-                BufferSize::Fixed(config.sample_rate / 10)
+                // PipeWire manages its own buffer scheduling via quantum;
+                // forcing a fixed buffer size here conflicts and causes underruns
+                // at high sample rates (192kHz)
+                BufferSize::Default
             },
         };