refactor: rename shell and organize source

[cekrauseee]

What changed

• Renamed the product and page title to shell, with $ as the visible prompt.
• Moved application code to src/app, src/features, and src/lib while keeping documentation at the repository root.
• Removed unused Next.js scaffold assets, the default favicon, and the empty generated Next config.
• Updated TypeScript path aliases and canonical documentation for the new structure.

Why

This prepares the project to evolve as an interactive shell instead of a personal portfolio, with a cleaner source layout for future features.

Validation

• npm run format:check
• npm run lint
• npm run typecheck
• npm run build
• Local browser verification: page loads with title shell, $ prompt, no error overlay, and no console errors.