Migrate to ESLint v9/10 (flat config) + @typescript-eslint v8

[DGINXREAL]

Background

Dependabot opened these bumps individually, but they can't merge in isolation (ERESOLVE peer conflicts) — they must be upgraded together as one coordinated change. Closed in favor of this tracking issue:

• build(deps-dev): bump eslint from 8.57.1 to 10.5.0 #16 — eslint 8.57.1 → 10.5.0
• build(deps-dev): bump @typescript-eslint/parser from 5.62.0 to 8.61.0 #3 — @typescript-eslint/parser 5.62.0 → 8.61.0

Current state (on main)

• eslint ^8.57.1
• @typescript-eslint/parser ^5.62.0
• @typescript-eslint/eslint-plugin ^5.62.0
• Legacy config: .eslintrc.json
• Lint script: eslint --ext .ts,.tsx .

Why it's blocked

• @vitejs/plugin-react@6 is already on main, but the ESLint stack is still v8.
• eslint@10 requires the flat config format and @typescript-eslint v8; the plugin and parser must move together (v8 pairs with eslint 9/10).

Scope of work

• Bump together: eslint, @typescript-eslint/parser, @typescript-eslint/eslint-plugin (v8), and any eslint plugins (e.g. react/react-hooks) to flat-config-compatible versions
• Convert .eslintrc.json → eslint.config.js (flat config); remove .eslintrc.json
• Drop the now-unsupported --ext flag from the lint script (flat config handles file matching via config)
• Run npm run lint and fix any new rule violations
• Verify npm ci, npm run lint, npm run typecheck, npm test all pass in CI

References

• ESLint flat config migration guide: https://eslint.org/docs/latest/use/configure/migration-guide
• typescript-eslint v8: https://typescript-eslint.io/blog/announcing-typescript-eslint-v8