Set up React SPA foundation with header, hero section, and core UI components

[mrsvolodya]

Summary

• React migration: Converted the project from vanilla JS to a React 19 SPA with React Router, Parcel bundler (with raw SVG/image transformers), and path aliases (@components, @assets,
  @hooks, @styles, @utils)
• Core UI components: Built reusable Button (polymorphic link/anchor/button), H (heading with size system), P (paragraph), ResponsiveImage (picture with mobile/tablet/desktop
  sources), and LanguageSwitcher (EN/UA dropdown)
• Layout system: Implemented Header (with main/sidemenu variants), SideMenu (full-screen slide-in navigation), MainLayout wrapper, and LandingHomeBlock hero section with
  responsive hero images (PNG + WebP)
• i18n: Integrated i18next with EN/UA translation files and language switcher
• Design system: Established SCSS architecture with BEM naming, responsive mixins (mobile/tablet/desktop), spacing scale, color variables, typography (Playfair Display + Raleway), and
  shared extends/placeholders
• Custom hooks: Added useScreenSize with debounced resize listener

New dependencies

react, react-dom, react-router-dom, i18next, react-i18next, chalk

Test plan

• Verify npm start runs the dev server without errors
• Confirm responsive layout at mobile (<640px), tablet (640-1279px), and desktop (1280px+) breakpoints
• Test language switcher toggles between EN and UA
• Test side menu opens/closes correctly
• Verify hero images load correct format (WebP with PNG fallback) per breakpoint
• Run npm run lint with no errors