Feature description

Improve secret key validation for all import formats (.env, .json, .yml) to prevent users from creating secrets with Unicode or special-character keys.
Only allow ASCII-safe keys:
A–Z a–z 0–9 . _ -

Invalid keys should be rejected or highlighted with a tooltip warning.

Why would it be useful?

Unicode and special characters in secret keys are not supported by most platforms, including:

AWS Lambda / ECS / Parameter Store
Docker
Kubernetes env vars
GitHub Actions
Vercel / Netlify / Render

These keys cannot be mapped into environment variables and often cause deployment failures or missing secrets.
Validating them early prevents confusing production issues.

Additional context

• Should prevent users from including JSON or .env files that contain invalid keys in the "Paste Secret Values" modal.

• When users enter an invalid key in the "Add secret key" modal, a warning should be displayed to let the user correct it.