fix(gui): properly handle AST Inspector popup state

- Add _show_ast_inspector flag to track when popup should open
- Use same pattern as other modals (_show_* flag + open_popup)
- Restructure if/else to properly handle end_popup paths
- This fixes the Inspect button not opening the modal

config.toml

@@ -56,7 +56,7 @@ Theme = true
 Diagnostics = false
 "External Tools" = false
 "Shader Editor" = false
-"Undo/Redo History" = true
+"Undo/Redo History" = false
 
 [theme]
 palette = "Nord Dark"