fix: stabilize web loading and paper rendering

src/app/document.rs

@@ -228,6 +228,7 @@ pub(super) struct DocumentTab {
     pub(super) last_synced_camera_gen: u64,
     /// Render-state key of `scene.document.preview`. Matching saves reuse the
     /// encoded DWG thumbnail instead of rescanning every resident wire.
+    #[cfg(not(target_arch = "wasm32"))]
     pub(super) thumbnail_cache_key: Option<super::ThumbnailCacheKey>,
     /// Sentinel "Welcome / Start" tab. Always at index 0 when present.
     /// Cannot be closed; the viewport area renders a welcome page instead
@@ -463,6 +464,7 @@ impl DocumentTab {
             block_edit: None,
             active_mleader_style: "Standard".to_string(),
             last_synced_camera_gen: 0,
+            #[cfg(not(target_arch = "wasm32"))]
             thumbnail_cache_key: None,
             is_start: false,
             pan_mode: false,