web: autosave snapshots with crash-recovery restore, persistent storage

The web build had no autosave: on_autosave was a no-op and the
subscription was native-only. Both gates are gone; every savetime tick
now serializes each dirty tab (same pre-serialize sync as the web save
path, no thumbnail, serialized serially off the critical paint path)
into a dedicated opencadstudio-autosave OPFS directory, keyed by
drawing name, without touching the dirty flag, downloads, or recents.

On boot the web app scans that directory and offers any snapshot
newer than the last saved copy in a localized Recover Drawing modal
(Restore opens it as "name (recovered).dwg" through the standard
open pipeline; Discard removes it; Escape discards). Saving or
closing a drawing clears its snapshot; the boot scan also garbage-
collects stale entries. run_web now requests navigator.storage.persist
so settings and OPFS data survive browser eviction.

src/app/update/mod.rs

@@ -959,6 +959,17 @@ impl OpenCADStudio {
                 }
             },
 
+            #[cfg(target_arch = "wasm32")]
+            Message::WebAutosavesScanned(result) => self.on_web_autosaves_scanned(result),
+            #[cfg(target_arch = "wasm32")]
+            Message::WebAutosaveRestore => self.on_web_autosave_restore(),
+            #[cfg(target_arch = "wasm32")]
+            Message::WebAutosaveDiscard => self.on_web_autosave_discard(),
+            #[cfg(target_arch = "wasm32")]
+            Message::WebAutosaveBytes(name, result) => self.on_web_autosave_bytes(name, result),
+            #[cfg(target_arch = "wasm32")]
+            Message::WebAutosaveSnapshotFinished(result) => self.on_web_autosave_finished(result),
+
             Message::ImagePick => {
                 Task::perform(crate::io::pick_image_file(), Message::ImagePickResult)
             }
@@ -7054,6 +7065,12 @@ impl OpenCADStudio {
                 if self.active_modal == Some(super::ModalKind::RecoveryPrompt) {
                     return self.update(Message::RecoveryDecline);
                 }
+                // Closing the crash-recovery prompt (✕ or Escape) discards
+                // the offered snapshot — otherwise it would requeue itself.
+                #[cfg(target_arch = "wasm32")]
+                if self.active_modal == Some(super::ModalKind::AutosaveRestore) {
+                    return self.update(Message::WebAutosaveDiscard);
+                }
                 // Closing the shortcut editor with un-applied rows needs an
                 // explicit discard; a second close attempt (or the overlay's
                 // Discard button) goes through.