initial project setup with gitignore

ignores build outputs, node_modules, nix results, and legacy folders

.gitignore

@@ -0,0 +1,31 @@
+# Build outputs
+build/
+*.pdf
+*.html
+.dev-index/
+
+# Dependencies
+node_modules/
+
+# Editor
+.vscode/
+*.swp
+*.swo
+*~
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Nix
+result
+result-*
+.direnv/
+
+# Legacy project folders (to be removed after migration)
+hdm-dateiformate-slides/
+hdm-internettechnik-slides/
+
+# Temporary files
+*.tmp
+*.bak