fix(start): decode recent-file thumbnails off the boot path

load_config synchronously parsed every recent DWG's embedded preview
on the main thread before the first frame, so the Start page appeared
seconds late on DWG-heavy recents. refresh_recent_thumbs now decodes
the missing previews on a background thread and delivers them via
RecentThumbsLoaded; the page opens immediately and thumbnails pop in
as they resolve. The web build (no threads, no filesystem previews)
skips the fetch.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/recent.rs

@@ -12,23 +12,56 @@ pub(super) const RECENT_MAX: usize = 100;
 pub(super) const RECENT_DEFAULT: usize = 20;
 
 impl OpenCADStudio {
-    /// Record a freshly opened file at the top of the recents list.
-    pub(super) fn push_recent(&mut self, path: PathBuf) {
+    /// Record a freshly opened file at the top of the recents list. Returns
+    /// the background task that decodes its thumbnail.
+    pub(super) fn push_recent(&mut self, path: PathBuf) -> iced::Task<crate::app::Message> {
         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();
+        self.refresh_recent_thumbs()
     }
 
-    /// 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));
+    /// Decode any not-yet-cached DWG preview thumbnails for the current
+    /// recents on a background thread, delivering them via
+    /// [`Message::RecentThumbsLoaded`]. The synchronous version of this ran on
+    /// the boot path and stalled the first frame for as long as it took to
+    /// parse every recent DWG's preview — the Start page appeared seconds
+    /// late. Cached per path (a `None` result is cached too); safe to call
+    /// repeatedly.
+    pub(super) fn refresh_recent_thumbs(&mut self) -> iced::Task<crate::app::Message> {
+        let missing: Vec<std::path::PathBuf> = self
+            .recent_files
+            .iter()
+            .filter(|p| !self.recent_thumbs.contains_key(*p))
+            .cloned()
+            .collect();
+        if missing.is_empty() {
+            return iced::Task::none();
+        }
+        // The web build has no spawnable threads and no filesystem previews —
+        // recents there simply show without thumbnails.
+        #[cfg(target_arch = "wasm32")]
+        {
+            return iced::Task::none();
+        }
+        #[cfg(not(target_arch = "wasm32"))]
+        {
+            let (tx, rx) = iced::futures::channel::oneshot::channel();
+            std::thread::spawn(move || {
+                let thumbs: Vec<_> = missing
+                    .into_iter()
+                    .map(|p| {
+                        let h = crate::io::thumbnail::read_handle(&p);
+                        (p, h)
+                    })
+                    .collect();
+                let _ = tx.send(thumbs);
+            });
+            iced::Task::perform(
+                async move { rx.await.unwrap_or_default() },
+                crate::app::Message::RecentThumbsLoaded,
+            )
         }
     }