fix(web): gate native-only code out of the wasm build

The wasm check broke on the MCP `tools` op, the block library's folder
picker and DWG thumbnails, and the PDF export-window culling helpers.
Gate each one like the code around it; `read_handle` needs no gate.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/control/mod.rs

@@ -528,6 +528,7 @@ impl OpenCADStudio {
                 Task::none(),
             );
         }
+        #[cfg(not(target_arch = "wasm32"))]
         if op == "tools" {
             return (
                 json!({