Install: copy all staging root files via directory iteration

Made-with: Cursor

source/install.h

@@ -35,6 +35,7 @@ void install_check_and_clear_screen_if_needed(void);
 bool install_path_exists(const char *path);
 int install_count_directory_items(const char *path);
 void install_combine_path(char *result, size_t size, const char *base, const char *add);
+int install_copy_staging_root_files(const char *staging, const char *dst_root);
 int delete_path_list(const char* paths[], const char* description);
 int delete_path_lists_grouped(const char *folder_display_name, ...);
 int folder_delete_single_with_progress(const char *path, const char *display_name);