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]>

.gitignore

@@ -41,3 +41,6 @@ __pycache__/
 
 # Worktrees created for isolated runs
 .claude/worktrees/
+
+# Single-instance lock files the app writes next to an open drawing
+*.ocs.lock

src/app/update/file.rs

@@ -5119,8 +5119,8 @@ impl OpenCADStudio {
         d.apply_plot_styles = ps.flags.plot_plot_styles;
         d.show_plot_styles = ps.flags.show_plot_styles;
         d.style_missing = !d.style_name.is_empty() && !style_loaded;
-        self.refresh_custom_scale_fields();
         d.style_error = if d.style_missing { style_error } else { None };
+        self.refresh_custom_scale_fields();
     }
 
     /// Tell the user what a plot style table's loader had to tolerate.

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")]

tests/fixtures/plot/.page-setups-metric.dxf.ocs.lock

@@ -1 +0,0 @@
-Open CAD Studio pid=898490 path=/home/panos/OpenCADStudio/tests/fixtures/plot/page-setups-metric.dxf