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/update/blocks_palette.rs

@@ -77,6 +77,10 @@ impl OpenCADStudio {
             BlockPaletteMsg::ToggleOptions => {
                 self.block_palette.options_collapsed ^= true;
             }
+            // Browsers expose no folder picker.
+            #[cfg(target_arch = "wasm32")]
+            BlockPaletteMsg::LibraryBrowse => {}
+            #[cfg(not(target_arch = "wasm32"))]
             BlockPaletteMsg::LibraryBrowse => {
                 return Task::perform(
                     async {