remove imgui subtree and bring it in as cmake fetch content

also forgot to update nix flake for implot and plutosvg

src/chat_gui4.cpp

@@ -19,9 +19,9 @@
 #include <entt/entity/entity.hpp>
 #include <entt/entity/runtime_view.hpp>
 
-#include <imgui/imgui.h>
-#include <imgui/misc/cpp/imgui_stdlib.h>
-#include <imgui/imgui_internal.h>
+#include <imgui.h>
+#include <misc/cpp/imgui_stdlib.h>
+#include <imgui_internal.h>
 
 #include <SDL3/SDL.h>