Add env var support for static-dir and i18n-dir flags

[alanef]

Summary

• Adds support for LISTMONK_static_dir and LISTMONK_i18n_dir environment variables
• Allows configuring custom static/i18n directories without modifying the command line
• Useful for Docker deployments where you can't easily modify the command

Changes

• Modified env var parsing to convert single underscores to hyphens for top-level keys (CLI flags like static-dir)
• Nested config keys (containing dots) keep underscores (e.g., db.ssl_mode)

Examples

# These now work:
LISTMONK_static_dir=/custom/static
LISTMONK_i18n_dir=/custom/i18n

# Existing env vars still work:
LISTMONK_db__ssl_mode=disable

Test plan

• Tested locally with docker-compose
• Verified LISTMONK_db__ssl_mode still works (nested config)
• Verified LISTMONK_static_dir works (top-level CLI flag)