Replace wpctl volume control with SDL_mixer VolumeMusic

Remove all PipeWire/wpctl system calls (SetDefaultSinkVolume,
listSinkIDs, listSinks, SetAllSinkVolumes, parseSinks, parseSinkLine,
Sink struct). Volume is now managed entirely through SDL_mixer's
VolumeMusic(), making audio fully system-agnostic.

Changes:
- mixer.go: delete ~170 lines of wpctl code; add SetVolume() public
  API using mix.VolumeMusic(math.Round(level * MAX_VOLUME))
- Init/Play/SwitchDevice set volume inline (they already hold the mutex)
- Play() now uses stored currentVolume instead of hardcoded MAX_VOLUME
- AudioOptions: SetAllSinkVolumes -> SetVolume
- mixer_test.go: add TestSetVolume (validation + error path coverage)

internal/mixer/mixer_test.go

@@ -7,6 +7,39 @@ import (
 	"rhemawavetech.com/rhemawave/internal/mixer"
 )
 
+func TestSetVolume(t *testing.T) {
+	tests := []struct {
+		name    string
+		level   float64
+		wantErr bool
+	}{
+		{"zero volume", 0.0, false},
+		{"half volume", 0.5, false},
+		{"max volume", 1.0, false},
+		{"above range", 1.5, true},
+		{"negative", -0.1, true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := mixer.SetVolume(tt.level)
+			if tt.wantErr {
+				if err == nil {
+					t.Errorf("SetVolume(%v) = nil, want error", tt.level)
+				}
+				return
+			}
+			if err != nil {
+				// For valid input, failure is expected when SDL isn't initialized
+				// (that's fine — we're testing validation, not SDL integration)
+				if err.Error() != "audio mixer not initialized" {
+					t.Errorf("SetVolume(%v) = %v, want 'audio mixer not initialized' or nil", tt.level, err)
+				}
+			}
+		})
+	}
+}
+
 func TestConvertVolume(t *testing.T) {
 	tests := []struct {
 		name     string