fix: resolve unhandled rejections and testnet explorer URL (#420, #421)

[Teeeyanaa]

#420 — async load() threw on status=0 becoming unhandled rejections

• All four pages (wallet, withdraw, transactions, request/[id]) had: if (cause instanceof ApiError && cause.status === 0) throw cause inside an async function called as void load(), so the throw became an unhandled promise rejection. React error boundaries never saw it.
• wallet/page.tsx: instead of throwing, setError() is now called, so the page renders an error alert instead of the misleading 'Create payment address' panel during outages.
• withdraw/page.tsx: setError() + setBalances([]) so the spinner exits and the form is visible (with 0 balance) with the error above it.
• transactions/page.tsx: setError() so the existing ErrorState branch renders with a retry button.
• request/[id]/page.tsx: setError() so ErrorState renders on first load; the poll loop continues retrying on subsequent failures, which is the right behaviour for transient network gaps.

#421 — receipt links hardcoded to testnet; vercel.json used 'MAINNET'

• transactions/page.tsx: replaced the hardcoded testnet constant with: const STELLAR_NETWORK = process.env.NEXT_PUBLIC_STELLAR_NETWORK ?? 'TESTNET' const EXPLORER_SEGMENT = STELLAR_NETWORK === 'PUBLIC' ? 'public' : 'testnet' const EXPLORER_BASE = https://stellar.expert/explorer/${EXPLORER_SEGMENT}/tx On production (NEXT_PUBLIC_STELLAR_NETWORK=PUBLIC) receipt links now point at the mainnet explorer; local dev falls back to testnet.
• vercel.json: corrected 'MAINNET' → 'PUBLIC' in both env and build.env to match the 'PUBLIC' | 'TESTNET' | 'FUTURENET' union in lib/transfer-qr.ts. 'MAINNET' was never a valid value.

Summary

Closes #

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Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• 🚀 New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing behaviour to change)
• ♻️ Refactor (no functional change, code quality improvement)
• 🎨 UI / design update
• 📦 Dependency update
• 🔧 Configuration / tooling change
• 📝 Documentation update
• 🔒 Security fix

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Changes Made

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Testing

• Manual testing in local development (npm run dev)
• Unit tests added or updated (npm test)
• Existing tests still pass (npm test)
• Tested on Stellar Testnet (if blockchain changes are included)
• Tested with demo mode OFF (NEXT_PUBLIC_DEMO_MODE=false)
• Verified on mobile viewport (if UI changes are included)

Test steps for reviewers:

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Screenshots / Screen Recording

Before	After

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Checklist

Code Quality

• Code follows the project's style and conventions (ESLint passes: npm run lint)
• TypeScript types are correct (no new any without justification): npm run type-check
• No secrets, wallet keys, or PII committed to the repository
• New NEXT_PUBLIC_* variables added to .env.example (if applicable)

Observability

• New API routes use captureError / log from lib/observability
• New Stellar SDK calls are wrapped in try/catch forwarding to captureError
• No sensitive data (private keys, mnemonics, tokens) passed to log.* or captureError

Security

• User inputs are validated with Zod (or equivalent) before processing
• No new server-side secrets are exposed via NEXT_PUBLIC_* variables
• Webhook signatures are verified where applicable
• Rate limiting applied to new public API endpoints (Upstash)

Accessibility (UI changes only)

• Interactive elements have accessible labels (aria-label, aria-describedby)
• Colour contrast meets WCAG AA (4.5:1 for normal text)
• Keyboard navigation works for new interactive components

Documentation

• Inline code comments added for non-obvious logic
• README or docs updated if the change affects setup or configuration
• SECURITY.md updated if the change affects the security surface (rare)

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Deployment Notes

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Related Issues / PRs

closes #420
closes #421

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