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 <[email protected]>

src/app/mod.rs

@@ -227,6 +227,11 @@ pub(super) struct OpenCADStudio {
     ribbon: Ribbon,
     /// Recently opened files, newest first — backs the Start page panel.
     recent_files: Vec<std::path::PathBuf>,
+    /// Decoded DWG preview thumbnails for the Start page, keyed by path.
+    /// `None` = the file has no readable preview (DXF, missing, WMF). Filled
+    /// lazily by `refresh_recent_thumbs`; never read from a `view` call.
+    recent_thumbs:
+        std::collections::HashMap<std::path::PathBuf, Option<iced::widget::image::Handle>>,
     /// How many recent files to keep (user-set on the Start page, persisted).
     recent_limit: usize,
     /// Live text of the recent-limit input box (may differ from `recent_limit`
@@ -2090,6 +2095,7 @@ impl OpenCADStudio {
             // Populated from the consolidated config after construction
             // (`apply_config`); default empty here.
             recent_files: Vec::new(),
+            recent_thumbs: std::collections::HashMap::new(),
             recent_limit: recent::RECENT_DEFAULT,
             recent_limit_input: recent::RECENT_DEFAULT.to_string(),
             command_line: CommandLine::new(),

src/app/recent.rs

@@ -17,9 +17,21 @@ impl OpenCADStudio {
         self.recent_files.retain(|r| r != &path);
         self.recent_files.insert(0, path);
         self.recent_files.truncate(self.recent_limit);
+        self.refresh_recent_thumbs();
         self.save_config();
     }
 
+    /// Decode any not-yet-cached DWG preview thumbnails for the current recents
+    /// (cheap preview-only reads). Cached per path; safe to call repeatedly.
+    /// Call when the recents list changes — never from a `view`.
+    pub(super) fn refresh_recent_thumbs(&mut self) {
+        for path in self.recent_files.clone() {
+            self.recent_thumbs
+                .entry(path.clone())
+                .or_insert_with(|| crate::io::thumbnail::read_handle(&path));
+        }
+    }
+
     /// Drop a path from the recents list (manual removal from the Start page).
     pub(super) fn remove_recent(&mut self, path: &Path) {
         self.recent_files.retain(|r| r.as_path() != path);

src/app/update/file.rs

@@ -296,6 +296,7 @@ impl OpenCADStudio {
             .clamp(crate::app::recent::RECENT_MIN, crate::app::recent::RECENT_MAX);
         self.recent_files.truncate(self.recent_limit);
         self.recent_limit_input = self.recent_limit.to_string();
+        self.refresh_recent_thumbs();
         self.statusbar_config = cfg.statusbar;
         self.ribbon.set_collapse_mode(cfg.ribbon.collapse);
         self.plot_dialog = cfg.plot;
@@ -670,6 +671,19 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                 Task::none()
     }
 
+    /// Rasterize the current drawing into the document's DWG preview image, so
+    /// a saved file shows a thumbnail in file browsers and other CAD apps.
+    /// A degenerate/empty drawing clears any stale preview.
+    pub(super) fn stamp_thumbnail(&mut self, i: usize) {
+        self.tabs[i].scene.document.preview =
+            crate::io::thumbnail::from_scene(&self.tabs[i].scene);
+        // The on-disk thumbnail will change — drop the cached Start-page handle
+        // so it re-reads the updated file on the next refresh.
+        if let Some(p) = self.tabs[i].current_path.clone() {
+            self.recent_thumbs.remove(&p);
+        }
+    }
+
     pub(super) fn on_save_file(&mut self) -> Task<Message> {
                 if self.read_only {
                     self.command_line
@@ -695,6 +709,7 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                         crate::io::write_backup(&path);
                     }
                     self.sync_truck_solids_to_acis(i);
+                    self.stamp_thumbnail(i);
                     match crate::io::save(&self.tabs[i].scene.document, &path) {
                         Ok(()) => {
                             self.command_line
@@ -755,6 +770,7 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                 // Persist Ortho / running OSNAP into the header (Save-As path).
                 self.stamp_header_sysvars(i);
                 self.sync_truck_solids_to_acis(i);
+                self.stamp_thumbnail(i);
 
                 // Native: write to the chosen path. Web: download the bytes
                 // under the chosen name (no filesystem).

src/app/view/mod.rs

@@ -118,6 +118,7 @@ impl OpenCADStudio {
             start_page_view(
                 &self.patrons,
                 &self.recent_files,
+                &self.recent_thumbs,
                 self.recent_limit,
                 &self.recent_limit_input,
                 self.win_size.0,
@@ -2187,6 +2188,10 @@ pub(super) fn collapse_bar<'a>(name: &str, on_press: Message) -> Element<'a, Mes
 pub(super) fn start_page_view<'a>(
     patrons: &'a [(String, i64)],
     recents: &'a [std::path::PathBuf],
+    thumbs: &'a std::collections::HashMap<
+        std::path::PathBuf,
+        Option<iced::widget::image::Handle>,
+    >,
     recent_limit: usize,
     recent_limit_input: &'a str,
     avail_w: f32,
@@ -2540,7 +2545,7 @@ pub(super) fn start_page_view<'a>(
     let avail = (avail_w - 16.0).max(0.0); // minus the page's l/r padding
     let fits_all = avail >= recent_w + welcome_min + sup_w;
 
-    let recent = recent_files_panel(recents, recent_limit, recent_limit_input);
+    let recent = recent_files_panel(recents, thumbs, recent_limit, recent_limit_input);
     let welcome = container(content).width(Fill).height(Fill);
 
     // Right rail: Patreon supporters, fetched at boot. When the list is empty
@@ -2719,6 +2724,10 @@ pub(super) fn start_page_view<'a>(
 // open — entries persist across sessions.
 pub(super) fn recent_files_panel<'a>(
     recents: &'a [std::path::PathBuf],
+    thumbs: &'a std::collections::HashMap<
+        std::path::PathBuf,
+        Option<iced::widget::image::Handle>,
+    >,
     limit: usize,
     limit_input: &'a str,
 ) -> Element<'a, Message> {
@@ -2779,17 +2788,37 @@ pub(super) fn recent_files_panel<'a>(
                 .map(|p| p.to_string_lossy().into_owned())
                 .unwrap_or_default();
 
+            // Leading DWG preview thumbnail (fixed box keeps rows aligned even
+            // when a file has no readable preview).
+            let thumb: Element<'a, Message> = match thumbs.get(path).and_then(|o| o.as_ref()) {
+                Some(h) => container(
+                    iced::widget::image(h.clone())
+                        .content_fit(iced::ContentFit::Contain)
+                        .width(Fill)
+                        .height(Fill),
+                )
+                .width(46)
+                .height(34)
+                .into(),
+                None => iced::widget::Space::new().width(46).height(34).into(),
+            };
+
             let path_for_open = path.clone();
             let open_btn = button(
-                column![
-                    text(crate::ui::text_util::elide(&name, 32))
-                        .size(12)
-                        .color(TEXT),
-                    text(crate::ui::text_util::elide(&dir, 42))
-                        .size(10)
-                        .color(MUTED),
+                row![
+                    thumb,
+                    column![
+                        text(crate::ui::text_util::elide(&name, 28))
+                            .size(12)
+                            .color(TEXT),
+                        text(crate::ui::text_util::elide(&dir, 38))
+                            .size(10)
+                            .color(MUTED),
+                    ]
+                    .spacing(2),
                 ]
-                .spacing(2),
+                .spacing(8)
+                .align_y(iced::Center),
             )
             .on_press(Message::OpenRecent(path_for_open))
             .padding([6, 12])