Replace JSON config with .env files

- Add python-dotenv dependency for environment variable loading
- Replace params.json and params-local.json with .env files
- Add --local and --local-stacosys flags to makesite.py
- Update Makefile to auto-detect stacosys availability
- Document configuration and usage in README.md

Makefile

@@ -3,9 +3,13 @@
 # declare phony targets
 .PHONY: build test typecheck
 
-# run locally
+# run locally (uses --local-stacosys if stacosys is running on port 8100)
 site:
-	uv run python makesite.py --params params-local.json
+	@if curl -s --connect-timeout 1 http://127.0.0.1:8100/api > /dev/null 2>&1; then \
+		uv run python makesite.py --local-stacosys; \
+	else \
+		uv run python makesite.py --local; \
+	fi
 	cd _site && python -m SimpleHTTPServer 2> /dev/null || python3 -m http.server
 
 # run type checks