web: harden drop-open and autosave after adversarial review

Review of the drop-open and autosave commits surfaced one blocker and
five majors; all are fixed here, and two claims in the autosave commit
message are corrected by this one: tab close does NOT clear a snapshot
(matches native .sv$ semantics — saving, restoring, discarding, exit,
and the capped boot scan do), and serialization runs on the main
thread between awaits, not off the paint path.

- The restore prompt can no longer appear with no entries (empty
  "Recover Drawing" modal on every boot once the autosave dir exists).
- Restore no longer consumes the snapshot before the open succeeds:
  the recovered drawing goes through the guarded drop queue, and the
  snapshot is deleted only after the tab opens; cancel or parse
  failure keeps the only copy.
- Saving during an autosave round defers its snapshot cleanup until
  the round finishes, so a stale round can no longer resurrect a
  snapshot newer than the save.
- The boot scan no longer deletes snapshots by mtime comparison
  (re-opening a drawing refreshes the recent copy's mtime, which
  silently destroyed wanted snapshots); GC is now a 16-entry cap with
  oldest-first eviction.
- OPFS write failures abort the writable stream (a leaked lock could
  stall the same-key writer and disable autosave for the session), and
  DOMException errors now carry their name/message.
- Empty (crash-window) snapshots are never offered; an unreadable one
  warns and stays discardable instead of looping forever.
- Drop-open polish: reads start inside the handler and complete in
  drop order (legacy WebKit safety, deterministic multi-file order),
  dragenter is canceled too, the install guard only latches on
  success, non-drawing drops don't steal propagation, a lock-poison
  panic can't cross into JS, and re-dropping an open drawing no
  longer risks clobbering an in-flight open (it re-queues).
- The model-space sheet floor is per-axis (a thin 8x400 mm profile
  keeps its length) and centers content on the enlarged page.

(cherry picked from commit 3edbc5b7c8)

src/app/update/mod.rs

@@ -1101,6 +1101,13 @@ impl OpenCADStudio {
                     self.command_line
                         .push_info(crate::tf!("Open cancelled: \"{}\"", p.name).as_ref());
                 }
+                // A cancelled recovered open keeps its snapshot (re-offered
+                // next boot — it is the only copy); the remaining queued
+                // snapshots can still be offered this session.
+                #[cfg(target_arch = "wasm32")]
+                if self.web_autosave_consumed.take().is_some() {
+                    self.queue_next_web_autosave_prompt();
+                }
                 self.drain_pending_open()
             }
 
@@ -1108,7 +1115,13 @@ impl OpenCADStudio {
             Message::WebFileDropped(name, bytes) => {
                 // Bytes are already in hand (no picker round-trip), so this is
                 // `on_open_file`'s web branch with the real name and size on
-                // the overlay from the start.
+                // the overlay from the start. An open already in flight must
+                // not be clobbered (its bytes are gone once dequeued), so the
+                // pair goes back to the queue for the next poll tick.
+                if self.opening.is_some() {
+                    crate::io::web_drop::enqueue_open(name, bytes);
+                    return Task::none();
+                }
                 let state = std::sync::Arc::new(crate::io::OpenProgressState::new(
                     crate::app::OPEN_PHASE_READING,
                 ));
@@ -1135,6 +1148,22 @@ impl OpenCADStudio {
                 if self.opening.as_ref().map(|opening| opening.id) != Some(open_id) {
                     return Task::none();
                 }
+                // A crash-recovery open settling here: success deletes its
+                // OPFS snapshot (the only copy, kept until now); a parse
+                // failure keeps it for the next boot. Either way the next
+                // queued snapshot can be offered again.
+                if self
+                    .web_autosave_consumed
+                    .as_ref()
+                    .is_some_and(|(_, recorded)| *recorded == outcome.name)
+                {
+                    if outcome.result.is_ok() {
+                        self.web_autosave_opened(&outcome.name);
+                    } else {
+                        self.web_autosave_consumed = None;
+                        self.queue_next_web_autosave_prompt();
+                    }
+                }
                 if let Some(opening) = self.opening.as_mut() {
                     opening.name = outcome.name.clone();
                     opening.source_path = Some(std::path::PathBuf::from(&outcome.name));