Developer announcement: validation library moved into this monorepo

[jekabs-karklins]

This PR migrates the validation package into the local workspace under validation/. Both the backend and frontend now consume it via a local file dependency defined in package.json

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⚠️ One-time action required (after pulling develop)

After pulling the latest changes, remove existing dependencies to ensure the apps use the local validation package:

• Delete frontend/node_modules
• Delete backend/node_modules
• Run npm install from the repository root

If you skip this step, the existing installed packages may take precedence over the local workspace version.

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🛠 Development workflow

1. Make changes to the validation code in validation/src
2. Run npm install from the repository root

Your updates should then be reflected automatically in both the backend and frontend.