Normalize Windows drive paths for mounted roots

[Peter7896]

Closes #87

Summary:

• Normalize Windows drive-letter paths before allowed-directory validation, so C:\... and C:/... match mounted roots such as /C/... when the server runs in a Unix-style environment.
• Apply the same normalization to configured allowed directories and file:// roots.
• Add regression coverage for native Windows paths inside mounted roots and rejection outside the allowed root.

Validation:

• cargo fmt --all -- --check
• cargo test
• cargo clippy --all-targets --all-features -- -D warnings
• cargo check --tests --target x86_64-unknown-linux-gnu