feat: upgrade to Next.js 15, React 19, and migrate to ESLint 9 flat config

Major framework upgrades:
- Next.js 13 → 15.5.6
- React 18.2.0 → 19.2.0
- ESLint 8 → 9 with flat config migration

Dependency updates:
- @headlessui/react, date-fns, zod, true-myth, usehooks-ts
- @typescript-eslint packages to v8
- TypeScript to v5.9.3
- Prettier to v3.6.2

Breaking changes:
- Migrate from .eslintrc.json to eslint.config.mjs
- Remove deprecated swcMinify option from next.config.mjs
- Update all React type definitions

next.config.mjs

@@ -8,7 +8,6 @@
 /** @type {import("next").NextConfig} */
 const config = {
   reactStrictMode: true,
-  swcMinify: true,
   i18n: {
     locales: ["en"],
     defaultLocale: "en",