feat(thumbnail): embed a DWG preview on save, show it on the Start page

Generate a small wire-raster thumbnail when saving a DWG and embed it via
acadrust's preview API, so OCS drawings show a thumbnail in file browsers
and other CAD applications. Read the embedded preview back cheaply (file
header + preview bytes only, no full parse) to show a thumbnail per file
in the Start page's Recent Documents panel.

- io/thumbnail.rs: CPU rasterizer (current-layout wires -> BMP DIB) plus
  a preview-only reader/decoder (BMP/PNG -> iced handle) for the Start
  page.
- Embedded on save via stamp_thumbnail on both save paths.
- recent_thumbs cache filled on config load / recents change (never from
  a view); a thumbnail is drawn beside each recent file.

Requires acadrust at 3111694 (preview read/write), already pinned in
Cargo.lock.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.lock

@@ -72,7 +72,7 @@ checksum = "366ffbaa4442f4684d91e2cd7c5ea7c4ed8add41959a31447066e279e432b618"
 [[package]]
 name = "acadrust"
 version = "0.4.0"
-source = "git+https://github.com/HakanSeven12/acadrust?branch=main#716d4b41c99e9540deb1fe009223d09b82fdfca9"
+source = "git+https://github.com/HakanSeven12/acadrust?branch=main#3111694c734bf6d86f1c3693e98657733070a277"
 dependencies = [
  "ahash 0.8.12",
  "anyhow",