fix: prevent Play() deadlock when device is not open

Play() held mu then called SwitchDevice() which also tried to acquire mu,
causing a deadlock. Replace with direct call to openDeviceLocked() which
does not lock.

Add regression test that verifies Play() returns an error (not deadlock)
when called on an uninitialized mixer.

internal/mixer/mixer_internal_test.go

@@ -2,8 +2,45 @@ package mixer
 
 import (
 	"testing"
+	"time"
+
+	"rhemawavetech.com/rhemawave/internal/db"
 )
 
+// TestPlayDoesNotDeadlockWhenUninitialized verifies that Play() returns an error
+// (not a deadlock) when the mixer has not been initialized.
+// The old code would deadlock because Play() held mu then called SwitchDevice()
+// which also tried to acquire mu.
+func TestPlayDoesNotDeadlockWhenUninitialized(t *testing.T) {
+	// Ensure mixer is in uninitialized state
+	mu.Lock()
+	wasInited := inited
+	inited = false
+	deviceOpen = false
+	mu.Unlock()
+
+	defer func() {
+		mu.Lock()
+		inited = wasInited
+		mu.Unlock()
+	}()
+
+	cfg := &db.AppConfig{AudioEnabled: true}
+	done := make(chan error, 1)
+	go func() {
+		done <- Play("nonexistent.wav", false, cfg)
+	}()
+
+	select {
+	case err := <-done:
+		if err == nil {
+			t.Fatal("Play() returned nil error on uninitialized mixer, want error")
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("Play() deadlocked on uninitialized mixer (did not return within 5s)")
+	}
+}
+
 func TestParseSinkLine(t *testing.T) {
 	tests := []struct {
 		name     string