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>

src/app/command_driver.rs

@@ -711,8 +711,12 @@ impl OpenCADStudio {
             let toks: Vec<String> = tokens.iter().map(|s| s.to_string()).collect();
             return self.finish_active_command(&toks, finish);
         }
+        // Bare XREF opens the External References palette, so probing the
+        // verb alone would open it (and refresh every entry) before the
+        // argument form `XREF Reload A` runs.
         if tokens[0].eq_ignore_ascii_case("BACKGROUND")
             || tokens[0].eq_ignore_ascii_case("COLORSCHEME")
+            || tokens[0].eq_ignore_ascii_case("XREF")
         {
             return self.dispatch_command(cmd);
         }