ifcviewer: load rdb/ifc as property data source on sidecar hit

Sidecar hits skipped opening the underlying .rdb/.ifc, so ifcFile() was
null and the property panel only showed cached name/type/guid. Now, after
a sidecar hit, a background thread opens <stem>.rdb (preferred) or
<stem>.ifc and hands the file to GeometryStreamer via setIfcFile(), with
a dataSourceReady signal so the UI refreshes the current selection.

Gated behind a new AppSettings::loadDataSource toggle (default on) so
users can opt into geometry-only viewing; when off, the sidecar-hit
thread is skipped and the stream-path ifc_file_ is released after
the sidecar write completes.

Also adds *.ifcview to the Add Files dialog filter so a cache can be
opened directly without its source file present.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer-full/SettingsWindow.h

@@ -44,6 +44,7 @@ private:
     QLineEdit* geometry_library_edit_ = nullptr;
     QCheckBox* show_stats_check_ = nullptr;
     QCheckBox* backface_culling_check_ = nullptr;
+    QCheckBox* load_data_source_check_ = nullptr;
 };
 
 #endif