Create `docs/architecture.md` for the frontend repo

[Marvell69]

New contributors need to understand the frontend's architecture decisions — why certain libraries were chosen, how the contract interaction layer is structured, and where to find things. An architecture doc saves repeated explanations in PR reviews.

Requirements and context

• Explain: folder structure and conventions, data flow (contract → hooks → components), wallet context design, React Query strategy, error handling approach, environment variable naming convention
• Include a simple architecture diagram
• Cover key trade-offs and decisions (e.g. why React Query over SWR, why sonner for toasts)
• Keep to 3–5 pages — prioritise clarity over exhaustiveness

Suggested execution

git checkout -b docs/architecture

• Create docs/architecture.md
• Include folder structure tree
• Add architecture diagram (ASCII or SVG)
• Link from README

Example commit message
docs: add frontend architecture overview documentation