feat: add Stellar payment instructions

[taherdhanera]

Closes #371

Claim

/claim #371

Summary

• Add StellarPaymentInstructions for the post-purchase Stellar payment flow: destination address, memo warning, amount, copy controls, QR code, countdown, expired state, and retry action.
• Add useOrderStatus to poll GET /api/orders/:id every 5 seconds and redirect to /tickets with the required success toast when the order becomes PAID.
• Add qrcode.react and focused coverage for rendering, PAID redirect, retry-payment behavior, and related CI-unblock fixes.

Validation

• npm run lint -> passed with warnings only.
• npm test -> 25 test files passed, 242 tests passed; global coverage thresholds pass.
• npm run build -> passed.
• npm ls qrcode.react -> qrcode.react@4.2.0 installed.

Notes

• This PR keeps the implementation scoped to issue FE-xxx: feat: build Stellar payment instructions UI — address, memo QR, countdown, and auto-confirm #371 and does not include the ticket-transfer scope from FE-xxx: feat: implement complete ticket transfer UI — replace alert() stub with real flow #370.

[taherdhanera]

Pushed a small follow-up on commit f6c3b0e to tighten the user-facing payment copy against the issue text:

• Memo warning now matches the requested wording: "Required ? your payment will fail without the memo".
• PAID redirect toast now uses an ASCII-safe escaped constant while rendering the requested "?? Tickets issued! Check your email." message.

Revalidated after the update:

• npx vitest run src/__tests__/stellar-payment-instructions.test.tsx -> 3 tests passed
• npx eslint src/features/orders/components/StellarPaymentInstructions.tsx src/hooks/useOrderStatus.ts src/__tests__/stellar-payment-instructions.test.tsx -> passed
• git diff --check -> passed
• npm ls qrcode.react -> qrcode.react@4.2.0

PR #397 remains open, non-draft, and mergeable from my side.

[taherdhanera]

Updated this PR against the current main after the branch became dirty from recent upstream merges.

Current state:

• mergeability is now CLEAN / MERGEABLE
• /claim #371 and Closes #371 remain in the PR body
• local verification passed: npm run lint (0 errors; existing warnings only) and npm test (25 files, 242 tests)