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 <[email protected]>

docs/book/src/guide/files.md

@@ -52,6 +52,10 @@ cache with `cache: true` on warm saves; saving never rebuilds cold geometry just
 create one. Ordinary saves and browser downloads default to `cache: false`. An additional display-mesh cache remains opt-in
 with `preview: true`. Unsupported or mismatched caches are ignored. Browser opening
 still rebuilds synchronously.
+Warm saves with explicit `cache: true` can also include a small PNG thumbnail. It travels with the file, so
+Home can show the recent design without reconstructing it when no local picture
+is available. Missing or incompatible pictures use the usual thumbnail generation.
+The default `cache: false` omits the portable thumbnail too.
 The repository's `docs/design-container.md` describes limits and encoding details.
 
 Current designs use format 16, including physical materials defined in the design, the font of sketch text, sheet metal corner relief shapes, sketch face anchors on cylinder caps, sketch axis