.env file validator

Paste a .env file. Each problem is reported with its line, and the variables are exported as render.yaml `envVars`.

The file is read exactly as Bex's Heroku and Docker Compose converters read it. Findings name keys and lines only, never values, and secret-looking values are exported as `sync: false` so you enter them in the dashboard.

These tools run entirely in your browser. Nothing you paste or drop is uploaded, stored, or sent to analytics.