docs: add Storybook stories for all existing components

[rayeberechi]

Summary

There are currently no interactive component previews. Adding Storybook stories makes it easy for contributors and consumers to explore components without setting up a full app.

Implementation Steps

1. Install Storybook: npx storybook@latest init — choose the Vite + React builder. Discuss the dependency additions in this issue before merging.
2. Create stories under src/components/**/*.stories.tsx (co-located with components).
3. Required stories:
   • ConnectWalletButton — Default, Connecting, Connected states.
   • CopyableAddress — Truncated, Full.
   • AssetDisplay — Native XLM, Custom asset with issuer shown.
   • AssetBalanceList — Empty, Multiple assets.
   • TransactionStatusBadge — Pending, Success, Failed.
   • TransactionHistory — Loading, Empty, Populated.
   • SendPaymentForm — Default.
4. Add a storybook and build-storybook script to package.json.
5. Update CONTRIBUTING.md with instructions for running Storybook locally.

Definition of Done

• Storybook initialised and running (npm run storybook).
• All listed stories created.
• CONTRIBUTING.md updated.
• No TypeScript errors in story files.