Add Nix-based CI workflows

Co-authored-by: dichhead <[email protected]>

README.md

@@ -131,6 +131,26 @@ Change the language from the application settings. The browser version also uses
 
 ## Build from source
 
+### Nix
+
+The GitHub workflows use the repository flake for a reproducible Rust, web, and automation toolchain. To match CI locally:
+
+```bash
+git clone https://github.com/HakanSeven12/OpenCADStudio.git
+cd OpenCADStudio
+nix develop
+```
+
+From that shell, run the same commands CI does, for example:
+
+```bash
+cargo fmt --all --check
+cargo test --locked --workspace --lib
+trunk build --locked --release --public-url /app/ --dist dist/app --html-output index.html web-app.html
+python3 scripts/test_site.py
+python3 scripts/test_release.py
+```
+
 ### Desktop
 
 Requirements: