fix: accept UTF-8 BOM in dotenv files

[LouisDeconinck]

Summary

Ignore one optional leading UTF-8 BOM when parsing .env input, while preserving BOM characters inside values and adding LF/CRLF regression coverage.

Checks

• git diff --check
• cargo test -p sotto-cli dotenv and cargo fmt --all --check could not run locally because Cargo is not installed

Closes #203

[github-actions]

Thanks for your first pull request to Sotto, and welcome.

Two things that trip up newcomers:

• Every commit needs a Signed-off-by line under the Developer Certificate of Origin - PRs with unsigned commits cannot be merged. Sign off with git commit -s; if your branch already has unsigned commits, run git rebase --signoff main and push again.
• Please run cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings before pushing, and add tests for new behaviour (cargo test --workspace runs the suite).

The full guide is in CONTRIBUTING.md - a maintainer will review shortly.

[Maxerns]

Since you've been on a roll: I opened #216 for the greeter posting "first PR" greetings on every PR from returning contributors (you've received a few yourself). It's a small Actions/J
avaScript task with a defined policy feel free to pick it up if it interests you.