fix: add secret directory, fix typo, bump node image to 2.4.0

[beamuu]

Summary

• Missing directory in repo: secret/ and secret/keystore/ are bind-mounted by all docker-compose files but were absent from the repo, causing containers to fail on first run after a fresh clone
• Typo in directory name: serect → secret across all volume mounts, entrypoint scripts, READMEs, and .gitignore (mainnet & testnet)
• Missing setup step in docs: READMEs jumped straight from clone to docker compose up with no mention of the required directory creation
• Outdated image version: bumped bitkubchainorg/node from 2.3.0 to 2.4.0 in all six docker-compose files

Changes

• Rename serect/ → secret/ in all compose files, entrypoint scripts, READMEs, and .gitignore for both mainnet/ and testnet/
• Add secret/.gitkeep and secret/keystore/.gitkeep so the required directory structure exists after a fresh clone
• Update .gitignore to track the empty directory structure while still ignoring actual secret files (password, keystore JSON)
• Add a Setup section to both READMEs with the mkdir -p secret/keystore step
• Bump node image from 2.3.0 → 2.4.0 in all six docker-compose files (mainnet + testnet: fullnode, validator, archivenode)

Test plan

• Fresh clone → cd mainnet → docker compose -f docker-compose.fullnode.yaml up -d starts without bind-mount errors
• secret/password.txt and keystore files generated at runtime are still gitignored
• Same applies to testnet

🤖 Generated with Claude Code

[beamuu]

Note: The bitkubchainorg/node:2.4.0 image has not been published to Docker Hub yet. This PR will be ready to merge once the image is available.