chore: add ESLint tooling with TypeScript support

The repository has a pnpm lint script running 'eslint .' but ESLint
and typescript-eslint were not listed in devDependencies.

Added:
- eslint (^9.0.0)
- @eslint/js (^9.0.0)
- typescript-eslint (^8.0.0)

Added eslint.config.mjs with flat config (ESLint 9+) using
typescript-eslint strict rules. Ignores node_modules, dist, build,
and framework-specific output directories.