Wire up ESLint + Prettier, fix existing violations, add CI workflow

[Du7chManiac]

• Add root-level ESLint flat config (eslint.config.js) with:
  • Base JS recommended rules
  • Node globals for backend; browser globals for frontend
  • React + React-Hooks (rules-of-hooks, exhaustive-deps) for frontend
  • React-Refresh (warning) for Vite HMR safety
  • Vitest globals for test files
  • eslint-config-prettier to avoid stylistic rule conflicts
• Add Prettier config (.prettierrc.json) matching existing code style
  (single quotes, semis, 2-space, 100-col) and .prettierignore for
  generated output (dist, public, drizzle, db files, lockfile)
• Add lint/lint:fix/format/format:check scripts to root package.json
• Install devDeps: eslint, @eslint/js, globals, eslint-plugin-react,
  eslint-plugin-react-hooks, eslint-plugin-react-refresh, prettier,
  eslint-config-prettier
• Mark root package.json as "type": "module" so eslint.config.js loads
  without the ESM warning
• Apply prettier --write across the repo (mechanical formatting only)
• Fix the 11 pre-existing ESLint errors surfaced by the new config:
  • Prefix unused args with _ (Express error middleware, carrier adapter
    contract params) or drop unused catch bindings / imports
  • Escape apostrophes in JSX text with '
• Add .github/workflows/ci.yml running lint, format:check, test, and
  build on PRs and pushes to main
• Update .github/copilot-instructions.md to document the new commands
  (the previous note about eslint being unavailable is no longer true)