core: implement XDR transaction builder with fee estimation

[EmeditWeb]

Problem

The API builds unsigned XDR transactions but does
not estimate fees correctly. Transactions with
insufficient fees are rejected by Stellar.

What To Build

1. Create src/stellar/transaction-builder.service.ts
2. Fee estimation from Horizon fee_stats
3. Fee bump transaction support
4. Retry logic for fee-related failures

Files To Touch

• src/stellar/transaction-builder.service.ts (new)
• src/stellar/stellar.service.ts
• All service files that build XDR

Acceptance Criteria

• Fee estimation calls Horizon fee_stats
• All XDR builders use estimated fee
• Fee bump transactions supported
• Insufficient fee errors trigger retry
• npm run build passes

Mandatory Checks Before PR

• npm run build passes
• No any types
• PR references this issue