Security: move private keys out of docker-compose.yaml

[Zena-park]

Problem

generateRemoteTestnetComposeFile in compose-generator.js embeds private keys directly into docker-compose.yaml as environment variables (e.g., ETHREX_DEPLOYER_L1_PRIVATE_KEY, ETHREX_BRIDGE_OWNER_PK).

This leaves secrets at rest on disk and easily retrievable via docker inspect or container logs.

Proposed Solution

• Use a separate .env file with strict permissions (chmod 600) instead of inlining keys in the compose file
• Or use Docker secrets for sensitive values
• Consider remote secret manager integration for production deployments

Context

Raised in PR #44 code review: #44 (comment)

Files

• crates/desktop-app/local-server/lib/compose-generator.js — generateRemoteTestnetComposeFile()