Add bounded portable Home thumbnails to design files (#518)

A save with cache: true also stores a small bounded PNG thumbnail (240 × 150, at most 256 KiB, keyed to the exact document) made from cached meshes only. Home uses it when its local picture is missing, without rebuilding the design; ordinary saves and downloads still omit it. Closes #303.

Co-authored-by: lillith <35337584+0xlillith@users.noreply.github.com>

crates/io/src/lib.rs

@@ -10,9 +10,10 @@ mod design;
 mod design_metadata;
 mod dxf;
 pub use design::{
-    DesignCompression, DesignContainerReader, DesignGeometry, DesignPreview, is_design_container, read_design_file, read_design_geometry,
-    read_design_preview, write_design_container, write_design_container_with_cache, write_design_container_with_compression,
-    write_design_container_with_preview,
+    DesignCompression, DesignContainerReader, DesignGeometry, DesignPreview, THUMBNAIL_SIZE, is_design_container, read_design_file,
+    read_design_geometry, read_design_preview, read_design_thumbnail, read_design_thumbnail_file, write_design_container,
+    write_design_container_with_cache, write_design_container_with_compression, write_design_container_with_preview,
+    write_design_container_with_thumbnail,
 };
 mod legacy_dir;
 pub mod safe_write;