Add ESLint and Prettier configuration files for backend and frontend

[Josie123-Dev]

What

Create .eslintrc.js and .prettierrc configuration files for both the backend and frontend packages. Add lint-staged and husky pre-commit hooks to enforce formatting on commit.

Why

Both backend/package.json and frontend/package.json have lint and format scripts, but no actual ESLint or Prettier configuration files exist. This means:

• npm run lint either fails or uses unpredictable defaults
• Code style is not enforced across contributors
• PRs can introduce inconsistent formatting without any check catching it

The smart contracts enforce cargo fmt and cargo clippy in CI with -D warnings. The JS packages should have equivalent enforcement.

Scope

In scope:

• backend/.eslintrc.js — NestJS-appropriate config (extends @typescript-eslint/recommended)
• backend/.prettierrc — consistent formatting rules
• frontend/.eslintrc.js — Next.js-appropriate config (extends next/core-web-vitals)
• frontend/.prettierrc — consistent formatting rules
• lint-staged + husky pre-commit hook (optional, in root or per-package)
• Fix any existing lint errors so npm run lint passes cleanly

Out of scope:

• Monorepo-level ESLint config (no root package.json yet)
• Custom ESLint plugins
• Refactoring code to fix lint warnings (only fix errors)

Acceptance Criteria

• backend/.eslintrc.js exists and npm run lint passes in backend/
• backend/.prettierrc exists and npm run format produces consistent output
• frontend/.eslintrc.js exists and npm run lint passes in frontend/
• frontend/.prettierrc exists and npm run format produces consistent output
• Both configs use consistent rules (same print width, tab width, trailing commas)
• Pre-commit hook runs lint-staged on staged files (if husky is added)

Technical Context

• Backend deps already installed: eslint, prettier, @typescript-eslint/eslint-plugin, @typescript-eslint/parser
• Frontend deps already installed: eslint, eslint-config-next
• NestJS recommended ESLint config: https://docs.nestjs.com/linting
• Frontend should extend next/core-web-vitals and @typescript-eslint/recommended

[Viccodes11]

I am confident that I can implement this having worked on something similar in the past