Document the Next.js app/ folder structure and overall architecture

[AlAfiz]

Description

The Next.js 14 App Router introduces new concepts like Server Components, Layouts, and Route Handlers. We need a map of the codebase so contributors know exactly where to put new files.

Acceptance Criteria

• Create an ARCHITECTURE.md file (or a dedicated section in the README).
• Explain the difference between the app/ directory (routing) and the components/ directory (UI).
• Define the rule for when to use 'use client' vs leaving a component as a Server Component.
• Map out where Soroban contract bindings, utility functions, and global state providers live.

Technical Details

• Include a simple ASCII tree diagram of the folder structure to make it visually digestible.