feat: add manual numeric inputs for fuzzy modeling and update env config

- Implement synchronized numeric inputs alongside sliders in Step 2.
- Enable real-time graph updates via input arrows and manual typing.
- Add strict clamping and validation for fuzzy parameter constraints.
- Refactor environment variables to use .env.local pattern.
- Add .env.example files for frontend and backend documentation.

frontend/.gitignore

@@ -12,6 +12,11 @@ dist
 dist-ssr
 *.local
 
+# Variables de entorno locales de Vite
+# (.env y .env.example sí se versionan)
+.env.local
+.env.*.local
+
 # Editor directories and files
 .vscode/*
 !.vscode/extensions.json