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/GeometryStreamer.cpp

@@ -79,6 +79,10 @@ GeometryStreamer::~GeometryStreamer() {
     }
 }
 
+void GeometryStreamer::setIfcFile(std::unique_ptr<ifcopenshell::file> file) {
+    ifc_file_ = std::move(file);
+}
+
 void GeometryStreamer::loadFile(const std::string& path, uint32_t start_object_id, uint32_t model_id, int num_threads) {
     if (running_.load()) {
         cancel();