fix(audio): suppress audio:error toast for superseded plays

Rapid skipping while a track is in initial preparation produced a
misleading "Couldn't play track — skipping" toast for the abandoned
track. Sequence: audio_play(A) reaches build_source_from_play_input;
user skips → audio_play(B) bumps gen; A's RangedHttpSource::read sees
the gen mismatch and returns Ok(0); Symphonia's probe interprets Ok(0)
as EOF → "ranged-stream: format probe failed: end of stream";
audio_play(A)'s map_err emits audio:error unconditionally → toast
appears even though playback already moved on.

Gate the emit on a generation check: if the global generation has
already moved past this play's gen, the failure is supersedion, not a
real codec error — log it but do not surface the toast.

Pre-existing bug, identical code path on main (commands.rs:600). Noticed
on refactor/backend-pilot during cucadmuh's skip-test session because
the live-test pattern hit the narrow probe-window race repeatedly.

The diagnostic logs added in the previous commit should make the next
occurrence's cause visible regardless of whether the toast fires.

Frontend impact:
- handleAudioError no longer fires on supersedion → no toast, no
  setState({ isPlaying: false }), no queued next() call.
- The audio_play IPC promise still rejects with the same error string
  (preserved as the .map_err return value); the JS .catch is already
  generation-guarded in playerStore so this is a no-op there.

src-tauri/src/audio/commands.rs

@@ -240,7 +240,19 @@ pub async fn audio_play(
     )
     .await
     .map_err(|e| {
-        app.emit("audio:error", &e).ok();
+        // Suppress the audio:error toast when this play was already superseded
+        // by a newer audio_play (rapid skip): the failure is the inevitable
+        // Ok(0)/EOF from RangedHttpSource after gen-bump, not a real codec
+        // problem. The frontend would otherwise show "Couldn't play track" for
+        // the abandoned URL while a new track is already loading.
+        if state.generation.load(Ordering::SeqCst) == gen {
+            app.emit("audio:error", &e).ok();
+        } else {
+            crate::app_deprintln!(
+                "[audio] suppressed audio:error for superseded play (gen={} cur={}): {}",
+                gen, state.generation.load(Ordering::SeqCst), e
+            );
+        }
         e
     })?;
     state.current_is_seekable.store(playback_source.is_seekable, Ordering::SeqCst);