fix(electrobun): set GTK window background to black (prevents white host flash)

agor-pty/native/agor_resize.c

@@ -211,6 +211,13 @@ void agor_resize_init(void *window_ptr, int border)
         webview = child;
     }
 
+    /* Set GTK window background to black (prevents white flash before WebView loads) */
+    {
+        GdkRGBA black = { 0.0, 0.0, 0.0, 1.0 };
+        gtk_widget_override_background_color(GTK_WIDGET(stored_window), GTK_STATE_FLAG_NORMAL, &black);
+        fprintf(stderr, "[agor-resize] Window background set to black\n");
+    }
+
     stored_webview = webview;
     if (webview == GTK_WIDGET(stored_window)) {
         fprintf(stderr, "[agor-resize] WARNING: no child widget found, connecting on window\n");