feat(plot): PSETUPIN and page-setup import, page setup for new layouts

- PSETUPIN / -PSETUPIN: with no arguments a file picker offers the chosen
  drawing's named page setups in the Plot dialog; `PSETUPIN <file>` opens
  the same chooser on that file; `PSETUPIN <file> *` or
  `PSETUPIN <file> name,name` import straight away. Same-named setups are
  replaced, the import is one undo step, and the report counts both.
- The Plot dialog's page-setup list gains "Import…": the chooser shows the
  source drawing's setups with a tick each, All, Import and Cancel, and
  selects the first imported setup afterwards. The source is read off the
  UI thread with the same reader the editor uses for its own setups
  (`scene::document_page_setups`), and handles that only meant something
  in the source drawing (plot view, visual style) are dropped on import.
- Options → General → Plotting: "Show the page setup for new layouts"
  opens the Plot / Page Setup dialog whenever a layout is created
  (persisted with the plot preferences).

Tests: the document reader, the command (all / named / unknown / missing
file / undo) against a drawing saved by the app, the chooser flow, and the
new-layout option. Nine new strings in all 21 locales.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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

@@ -1 +1 @@
-Open CAD Studio pid=842697 path=/home/panos/OpenCADStudio/tests/fixtures/plot/page-setups-metric.dxf
+Open CAD Studio pid=854778 path=/home/panos/OpenCADStudio/tests/fixtures/plot/page-setups-metric.dxf