fix(grid-overlay): write JSON defaults at startup, not per-stream

Move set_nested_default calls from stream_model constructor to ux_window
startup so the viewer_model.grid section is written to realsense-config.json
on first launch even when no camera is connected.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Evgeni Raikhel

common/stream-model.cpp

@@ -37,13 +37,6 @@ namespace rs2
             namespace cfg = configurations::viewer::viewport_grid;
             auto& cf = config_file::instance();
 
-            cf.set_nested_default( cfg::horizontal_lines, 1   );
-            cf.set_nested_default( cfg::vertical_lines,   1   );
-            cf.set_nested_default( cfg::line_width,       1   );
-            cf.set_nested_default( cfg::line_color_r,     255 );
-            cf.set_nested_default( cfg::line_color_g,     255 );
-            cf.set_nested_default( cfg::line_color_b,     255 );
-
             auto valid_lines = []( int v ) { return ( v >= 1 && v <= 5 ) ? v : 1; };
             grid_h_lines    = valid_lines( cf.get_nested<int>( cfg::horizontal_lines, 1   ) );
             grid_v_lines    = valid_lines( cf.get_nested<int>( cfg::vertical_lines,   1   ) );

common/ux-window.cpp

@@ -87,6 +87,19 @@ namespace rs2
         config_file::instance().set_default(configurations::viewer::commands_xml, "./Commands.xml");
         config_file::instance().set_default(configurations::viewer::hwlogger_xml, "./HWLoggerEvents.xml");
 
+#ifdef BUILD_VIEWPORT_GRID_OVERLAY
+        {
+            namespace cfg = configurations::viewer::viewport_grid;
+            auto& cf = config_file::instance();
+            cf.set_nested_default( cfg::horizontal_lines, 1   );
+            cf.set_nested_default( cfg::vertical_lines,   1   );
+            cf.set_nested_default( cfg::line_width,       1   );
+            cf.set_nested_default( cfg::line_color_r,     255 );
+            cf.set_nested_default( cfg::line_color_g,     255 );
+            cf.set_nested_default( cfg::line_color_b,     255 );
+        }
+#endif
+
         std::string path;
         try
         {