fix(sheetset): read drawings off the UI thread

The wizard's folder scan, Import Layout as Sheet, the template layout
lists and the drawing cache behind Model Views, Place on Sheet, callout /
label blocks and block previews parsed whole DWGs on the UI thread. They
now go through the shared `background_task` worker: a cache miss reads the
drawing and handles the message that needed it again. Block previews lend
the cached drawing to the preview scene instead of cloning it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/update/mod.rs

@@ -96,6 +96,7 @@ mod blocks_palette;
 mod dialog;
 mod dynamic;
 mod file;
+pub(in crate::app) use file::background_task;
 mod page_setup_import;
 mod style;
 pub(in crate::app) mod util;