chore(SIGPG-7819): upgrade ESLint from v8 to v10 with flat config

[khamiltonuk]

What changed

• Upgraded ESLint from v8.57.0 to v10.2.0 (two major versions)
• Migrated from legacy .eslintrc.js to flat config (eslint.config.mjs)
• Replaced abandoned eslint-config-react-app@7.0.1 with direct plugin dependencies: typescript-eslint, eslint-plugin-react, eslint-plugin-react-hooks,
  eslint-plugin-jsx-a11y, eslint-plugin-import
• Added @eslint/compat to wrap plugins that use removed ESLint 10 context methods
• Removed --ext flag from lint script (removed in ESLint 9+)
• Bumped minimum Node from >=20.0.0 to >=20.19.0 (ESLint 10 requirement)
• Added vitest@1.6.0 as direct dev dependency
• All rules from eslint-config-react-app carried over; no effective rule changes

How to test

1. npm ci --legacy-peer-deps
2. npm run lint — 0 errors, 0 warnings
3. npm run test — 204 Cypress component tests pass
4. npm run test:ssr — 2 SSR vitest tests pass

Additional notes

• --legacy-peer-deps needed at install because some plugins haven't declared ESLint 10 peer deps yet (works fine at runtime)
• @eslint/compat wrappers can be removed once plugins update their APIs
• New tseslint.configs.recommended rules not in the old config (no-explicit-any, no-empty-object-type, no-require-imports) turned off to avoid
  introducing new noise