Add a CONTRIBUTING.md guide with local dev setup, code style, and PR checklist

[MJ-RWA]

Complexity: Medium — 150 pts

Summary

StellarFlow has a README but no CONTRIBUTING.md. New contributors — especially those coming in through Drip Wave — have no guidance on how to set up the dev environment, what the code conventions are, how to run tests, or what a PR needs to include to get merged. This issue creates a thorough CONTRIBUTING.md that makes the contributor experience smooth and reduces back-and-forth in PR reviews.

Files to Create/Modify

• Create: CONTRIBUTING.md at the repo root
• Modify: README.md — add a "Contributing" badge and link to CONTRIBUTING.md near the top

Required sections (in order)

1. Prerequisites — table of Node.js ≥18, npm ≥9, Git, Freighter (testnet), Bruno (optional)
2. Local Development Setup — fork/clone, npm install, npm run dev, fund a testnet account via Friendbot, add a USDC trustline for swap testing
3. Project Structure — annotated file tree with a "rule of thumb" for where new code goes (lib/stellar.ts for blockchain ops, pages/ for routes, components/ for shared UI, hooks/ for data fetching)
4. Code Style & Conventions — TypeScript-only, named vs default exports, Tailwind-only styling with the existing design tokens, try/catch + toast.error() for all async errors, no hardcoded addresses, import type for type-only imports, 28-byte memo enforcement
5. Working with the Stellar SDK — the standard "load account → build tx → return XDR" pattern, why submitTx from stellar.ts must be used instead of server.submitTransaction, and a Horizon error code reference table (op_underfunded, op_no_trust, op_no_destination, tx_bad_seq)
6. Testing Your Changes — manual checklist (build passes, lint passes, tested in Chrome+Freighter testnet, tested at 390px mobile width, no console errors) plus feature-specific test notes for Send, Swap, Recurring, and Request/Pay
7. Submitting a PR — branch naming conventions (feat/, fix/, docs/, refactor/), Conventional Commits format, and a full PR description template with Closes #XX
8. PR Checklist — at least 12 items covering issue linking, build/lint, screen recording, hardcoded addresses, error states, mobile responsiveness, Tailwind tokens, console.log cleanup, route/nav registration, hook cleanup, and submitTx usage
9. Getting Help — GitHub Discussions, GitHub Issues, Stellar Developer Discord, Stellar docs, Horizon API reference

Acceptance Criteria

• CONTRIBUTING.md exists at the repo root
• All 9 sections present: Prerequisites, Setup, Structure, Code Style, SDK guide, Testing, PR submission, Checklist, Help
• Local dev setup steps work exactly as written (tested on a fresh clone)
• Code style rules match the actual patterns in the existing codebase
• Horizon error code table is accurate
• Branch naming and commit message conventions are clearly defined
• PR description template is included and uses GitHub's issue-linking syntax (Closes #XX)
• PR checklist has at least 12 items covering build, lint, screen recording, mobile, and Stellar-specific rules
• README.md updated to link to CONTRIBUTING.md
• No broken links in the document

Screen Recording Requirements

1. A fresh clone of the repo — run npm install and npm run dev exactly as the guide describes
2. The app opens at http://localhost:5173 without errors
3. Scroll through CONTRIBUTING.md rendered on GitHub to show all sections are present and formatted
4. Show the PR template by opening a draft PR on GitHub — confirm the template auto-fills