Preserve invoice memos for internal Lightning payments

[pretyflaco]

Summary

• Persist wallet invoice descriptions so invoice-created memos/comments survive beyond BOLT11 decoding.
• Use the persisted invoice description when internal Blink-to-Blink Lightning payments settle via intraledger.
• Remove memo-sharing amount thresholds so transaction memos are no longer hidden based on payment size.

Related

Fixes #550
Related to #32

Test plan

• pnpm tsc-check
• pnpm test:unit -- tx-history.spec.ts wallet-invoice-builder.spec.ts
• TEST=send-lightning JEST_TIMEOUT=120000 pnpm test:integration