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 <noreply@anthropic.com>

.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