setup: add Prettier and connect to ESLint for frontend

[Lewiskunta]

Implements Card 7: frontend code quality tooling.

Changes:

• Installed Prettier and eslint-config-prettier
• Added .prettierrc with consistent formatting rules
• Added .prettierignore
• Updated eslint.config.js to disable ESLint formatting rules
• Added format and format:check scripts to package.json
• Added format:check step to CI frontend job
• Formatted all existing src/ files with Prettier

After this PR:

• npm run lint - catches bugs and bad patterns
• npm run format - reformats code to match config
• npm run format:check - verifies formatting in CI