fix: restore SDL volume to MAX after device switch

Remove dead 'currentVolume' variable that was never written to, causing
SwitchDevice to always restore volume to 0 after switching audio devices.

- Remove currentVolume from package state
- SwitchDevice now sets mix.MAX_VOLUME after reopening device
- GetVolume128 now queries SDL directly instead of reading stale variable
- Add unit tests for parseSinks, parseSinkLine, and ConvertVolume
- Add changelog.txt tracking all audio bug fixes

internal/mixer/mixer_test.go

@@ -19,6 +19,10 @@ func TestConvertVolume(t *testing.T) {
 		{"50 percent", 50, 0.50},
 		{"90 percent", 90, 0.90},
 		{"100 percent", 100, 1.0},
+		{"1 percent", 1, 0.01},
+		{"99 percent", 99, 0.99},
+		{"negative 10", -10, -0.10},
+		{"over 100", 150, 1.50},
 	}
 
 	const epsilon = 1e-9