FIX: debounce app_config->save() in wxEVT_IDLE handler (F-015)

The idle handler at GUI_App.cpp fired app_config->save() on every idle
event whenever the config was dirty. On Windows with a busy idle loop
this could produce dozens of synchronous disk writes per second.

Add a 5-second debounce using std::chrono::steady_clock so the flush
happens at most once every 5 s. Flush any remaining dirty state
unconditionally in OnExit() so no settings are lost on clean exit.

Ref: #10289

Author: tinezivic

src/slic3r/GUI/GUI_App.cpp

@@ -21,6 +21,7 @@
 #include <iterator>
 #include <exception>
 #include <cstdlib>
+#include <chrono>
 #include <regex>
 #include <thread>
 #include <string_view>
@@ -2774,6 +2775,10 @@ int GUI_App::OnExit()
         m_agent = nullptr;
     }
 
+    // Flush any config changes that were deferred by the idle-handler debounce.
+    if (app_config && app_config->dirty())
+        app_config->save();
+
     return wxApp::OnExit();
 }
 
@@ -3314,8 +3319,16 @@ bool GUI_App::on_init_inner()
         if (! plater_)
             return;
 
-        if (app_config->dirty())
-            app_config->save();
+        if (app_config->dirty()) {
+            // Debounce: avoid synchronous disk writes on every idle event.
+            // Save at most once per 5 seconds; OnExit() flushes any remaining dirty state.
+            static auto s_last_config_save = std::chrono::steady_clock::time_point{};
+            const auto now = std::chrono::steady_clock::now();
+            if (now - s_last_config_save >= std::chrono::seconds(5)) {
+                app_config->save();
+                s_last_config_save = now;
+            }
+        }
 
         // BBS
         //this->obj_manipul()->update_if_dirty();