Clearer error for an invalid --layer (lists the valid values)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GLUwWE2KmFPzhKaf67tWbx

src/wayland.rs

@@ -118,7 +118,7 @@ pub fn parse_layer(s: &str) -> Result<Layer> {
         "bottom" => Ok(Layer::Bottom),
         "top" => Ok(Layer::Top),
         "overlay" => Ok(Layer::Overlay),
-        other => bail!("unknown layer: {}", other),
+        other => bail!("unknown layer '{}' (valid: background, bottom, top, overlay)", other),
     }
 }