Wire IfcViewer to cloud sync connectors

Implements the viewer side of CLOUD_SYNC_PROTOCOL.md: connector
discovery, JSON-RPC stdio host, and Open/Save/Sync/Add cloud workflows
wired through the ribbon, Models panel right-click, and Settings tab.

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

src/ifcviewer-full/modules/models/AddModelDialog.h

@@ -30,6 +30,7 @@ enum class SourceMode {
     IfcFile,
     IfcDatabase,
     GeometryOnly,
+    CloudModel,
     ConvertToDatabase,
     ExportGeometryDatabase,
 };