refactor: Project structure improvements

[koistya]

• [x] Update the project structure to match Relay Starter Kit
• [x] Split route (page) components into sub-folders (settings, dashboard, auth, etc.)
• [x] Move the list of routes to app/routes/index.tsx
• [x] Move layout components to the app/layout folder
• [x] Introduce the primary (main) AppLayout and BaseLayout layout components
• [x] Move Material UI theme customizations to the app/theme folder
• [x] Update the login/signup pages
• [x] Use import.meta.env.VITE_{NAME} convention for environment variables (docs)
• [x] Upgrade all dependencies to the latest versions