Add controls for auto-generated mixed filament gradients

Add an app preference for mixed filament gradient auto-generation and
sync the manager state from startup and preferences changes.

Prompt before generating large gradient sets, preserve custom rows
when auto-generation is disabled, and refresh remapping/UI handling
accordingly. Add mixed filament coverage and a dumpinfo helper for
inspecting Windows minidumps.

src/libslic3r/AppConfig.cpp

@@ -321,6 +321,10 @@ void AppConfig::set_defaults()
         set_bool("auto_calculate_when_filament_change", true);
     }
 
+    if (get("auto_generate_gradients").empty()) {
+        set_bool("auto_generate_gradients", true);
+    }
+
     if (get("show_home_page").empty()) {
         set_bool("show_home_page", true);
     }