fix: drop the stray lock fixture; align merged plot code across targets

- tests/fixtures/plot/.page-setups-metric.dxf.ocs.lock was the app's own
  single-instance lock file, committed by accident in #1343; remove it and
  ignore *.ocs.lock.
- load_plotsettings_into_dialog: #1343's refresh_custom_scale_fields borrowed
  self while #1360 still wrote through the plot_dialog borrow; write the
  style error first.
- The wasm list_printers stub returns the Result #1358 gave the native one.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/io/print_to_printer.rs

@@ -56,8 +56,8 @@ pub struct PrintOptions {
 }
 
 #[cfg(target_arch = "wasm32")]
-pub fn list_printers() -> Vec<String> {
-    Vec::new()
+pub fn list_printers() -> Result<Vec<String>, String> {
+    Ok(Vec::new())
 }
 
 #[cfg(target_arch = "wasm32")]