remove imgui subtree and bring it in as cmake fetch content

also forgot to update nix flake for implot and plutosvg

src/theme.hpp

@@ -1,6 +1,6 @@
 #pragma once
 
-#include <imgui/imgui.h>
+#include <imgui.h>
 
 inline void setThemeGreen(void) {
 	ImVec4* colors = ImGui::GetStyle().Colors;