fix: show error banner when NEXT_PUBLIC_MOONPAY_API_KEY is unset (#178)

[JoesWalker]

PR Description:

Closes #178

Problem

When NEXT_PUBLIC_MOONPAY_API_KEY is not set, clicking "Buy Crypto (via MoonPay)" silently returned with no user feedback — no error, no toast, no
visual change.

Fix

• Added moonPayError state to components/dashboard/deposit.tsx
• handleMoonPayOpen now calls setMoonPayError(true) instead of silently returning when the API key is missing
• An inline error banner is rendered above the deposit method cards in both desktop and mobile views when the error state is set
• No new dependencies introduced — banner uses the existing red-tinted styling pattern consistent with the codebase
• Happy path (when key IS set) is unchanged

Testing

• With NEXT_PUBLIC_MOONPAY_API_KEY unset: clicking MoonPay shows the error banner
• With NEXT_PUBLIC_MOONPAY_API_KEY set: MoonPay opens normally, no banner shown
• TypeScript: tsc --noEmit passes with no errors

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