feat(desktop): replace audio stub with cpal backend and volume slider

- CpalAudioSink writes to SPSC ring buffer, cpal callback reads
- Graceful fallback to silent operation on audio device errors
- Volume slider (0-100%) in header bar with speaker icon
- Ring buffer cleared on ROM load and reset

crates/nesemu-desktop/Cargo.toml

@@ -7,3 +7,4 @@ edition = "2024"
 nesemu = { path = "../.." }
 gtk4 = "0.8"
 cairo-rs = "0.19"
+cpal = "0.15"