Fixed a rare crash on app startup caused by ENOTSUP (errno 45) on Mac (#14686)

fix: prevent startup crash when preset-sync directory scan hits a transient FS error

On startup the user-preset sync thread scans the preset folder for orphaned
.info files (scan_orphaned_info_files). It iterated the directory with a
throwing boost::filesystem::directory_iterator while running on a background
thread that has no exception guard. On macOS, readdir() can intermittently
fail with ENOTSUP (errno 45); boost then throws filesystem_error, which --
uncaught on the sync thread -- calls std::terminate and aborts the whole
application on startup.

- Iterate with the error_code-based directory_iterator so a transient read
  failure is logged and skipped instead of thrown. The orphan scan is
  best-effort and re-runs on the next sync, so skipping a cycle is harmless.
  This mirrors the existing pattern in has_json_presets() and the plugin scan.
- Wrap the entire sync-thread body in try/catch as defense-in-depth, so no
  future uncaught exception on that otherwise-unguarded thread can abort the
  app.

src/slic3r/GUI/GUI_App.cpp

@@ -6965,6 +6965,10 @@ void GUI_App::start_sync_user_preset(bool with_progress_dlg)
             // finishFn tears down the progress dialog (and clears the re-entrancy guard), so it
             // must run on every exit path — otherwise an early bail-out would leak the modal
             // dialog and leave the guard stuck, blocking all later manual syncs.
+            // Guard the whole thread body: an uncaught exception here (e.g. a transient
+            // boost::filesystem error while scanning the preset folder) would otherwise
+            // propagate out of the thread and terminate the entire application.
+            try {
             if (!m_agent) { finishFn(false); return; }
 
             // One-time scan for orphaned .info files left over from offline deletions; queues HTTP DELETEs.
@@ -7206,6 +7210,11 @@ void GUI_App::start_sync_user_preset(bool with_progress_dlg)
                     boost::this_thread::sleep_for(boost::chrono::milliseconds(500));
                 }
             }
+            } catch (const std::exception& e) {
+                BOOST_LOG_TRIVIAL(error) << "user preset sync thread terminated by exception: " << e.what();
+            } catch (...) {
+                BOOST_LOG_TRIVIAL(error) << "user preset sync thread terminated by unknown exception";
+            }
         });
 }
 
@@ -8526,8 +8535,13 @@ void GUI_App::scan_orphaned_info_files()
         if (!fs::exists(type_dir))
             continue;
 
-        // Iterate through all .info files
-        for (auto& entry : boost::filesystem::directory_iterator(type_dir)) {
+        // Iterate through all .info files. Use the error_code-based iterator so a transient
+        // directory-read failure (e.g. macOS readdir returning ENOTSUP) is logged and skipped
+        // instead of throwing an uncaught exception that would terminate the app from the
+        // background sync thread this runs on.
+        boost::system::error_code ec;
+        for (boost::filesystem::directory_iterator it(type_dir, ec), end; !ec && it != end; it.increment(ec)) {
+            const auto& entry = *it;
             if (entry.path().extension() != ".info")
                 continue;
 
@@ -8546,6 +8560,8 @@ void GUI_App::scan_orphaned_info_files()
                 }
             }
         }
+        if (ec)
+            BOOST_LOG_TRIVIAL(warning) << "scan_orphaned_info_files: failed to scan " << type_dir.string() << ": " << ec.message();
     }
 }