docs: add ARCHITECTURE.md overview guide

[Alhaji-naira]

Pull Request: Add ARCHITECTURE.md Developer & Onboarding Guide

Summary

This pull request introduces a comprehensive ARCHITECTURE.md guide in the repository's root directory. The document is designed to onboard new open-source contributors and establish consistent development patterns regarding Next.js 14 (App Router) paradigms and our Stellar/Soroban Web3 integration.

Key Sections Included

• Codebase Overview: A clean ASCII tree diagram mapping the folder hierarchy of the src/ directory to give developers an instant, bird's-eye view of the codebase.
• Separation of Concerns: Detailed guidelines explaining the boundary between the src/app/ folder (routing, page composition, layouts) and the src/components/ folder (modular and presentational UI).
• Server vs. Client Component Rules: Rules and best practices on when to leave components as default React Server Components (RSC) vs. when to use the 'use client' directive (interactivity, browser APIs, context consumers). It includes code snippets demonstrating both approaches.
• Web3 & Stellar/Soroban Integration Architecture: An architectural flow mapping the relationships between the UI, custom hooks (src/hooks), state stores (src/stores), contract bindings (src/soroban/contracts/), and the client/config files.

Benefits

• Improves developer onboarding velocity.
• Encourages consistent codebase patterns (proper rendering paradigms, structured folder usage).
• Ensures Web3/Soroban boundaries are clearly defined for new feature development.

closes #267