Tooling upgrades: ESLint, TypeScript, Next config

[tmshv]

Tasks (optional, do last)

1. Upgrade ESLint and TypeScript ESLint — package.json
@typescript-eslint/eslint-plugin is at v5 (outdated). Upgrade to v7+ and consider ESLint 9 flat config.

2. Raise tsconfig target — tsconfig.json:9
   Target is ES2017 but Node 20 supports ES2023+. Raise to ES2022 to avoid unnecessary downleveling.

3. Rename next.config.js → next.config.ts
   Next.js 15 supports TypeScript config natively. Provides type checking on the config object.