fix(xref): follow-ups to the xref attach parity merge

- `XREF <args>` runs whole: probing the bare verb first now opens the
  External References palette, which refreshed every entry and toggled
  the palette before `XREF Reload A` / `XREF ?` ran.
- Update the two tests that encoded the old behaviour: `-XREF ?` prints
  the reference table, and attaching a missing file is rejected.
- Drop the duplicate "Specify On-screen" catalog entry that shadowed the
  translated common key, with its unused ftl keys.
- Move `remove_pipe_symbols`' doc comment back above its function.
- Note in `document_for_save` that resolved xref content stays out.
- Gate the desktop-only preview cap and tab index for wasm.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

locales/ru-RU/opencadstudio.ftl

@@ -3502,7 +3502,6 @@ insert =
     .xref-attach-dialog-title = Attach External Reference
     .xref-attach-browse = Browse...
     .xref-attach-reference-type = Reference Type
-    .xref-attach-specify-on-screen = Specify On-screen
     .xref-attach-uniform-scale = Uniform Scale
     .xref-attach-insertion-point = Insertion point
     .xref-attach-angle = Angle: