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 <[email protected]>

src/scene/parametric_constraints.rs

@@ -2507,7 +2507,9 @@ impl super::Scene {
     }
 
     /// The document as written to a file: a dynamic dimension's screen-size
-    /// overrides are a display matter and stay out of the file.
+    /// overrides are a display matter and stay out of the file, and so does
+    /// what resolving an external reference merged in (its geometry, nested
+    /// blocks and styles), which the file never stores.
     pub(crate) fn document_for_save(&self) -> acadrust::CadDocument {
         use crate::entities::dim_override as ov;
         let mut document = self.document.clone();