fix: email case normalization and environment-dependent business URL

[rubenhensen]

Summary

• Normalize recipient email to lowercase before encryption — the wallet stores emails lowercased, so capitalized emails in the encryption policy caused silent decryption failures. See Email attribute is case-sensitive, causing decryption failures for capitalized emails #82.
• Make PostGuard for Business URL environment-dependent — reads BUSINESS_URL from runtime config (config.js / Terraform ConfigMap) so staging points to business.staging.postguard.eu and production to business.postguard.eu.

Test plan

• Encrypt a file for a recipient with mixed-case email (e.g. Alice@Example.com), verify decryption succeeds
• Verify nav and footer business links point to staging URL in staging environment
• Verify business links fall back to business.postguard.eu when BUSINESS_URL is not set in config