feat: add isolated lending pair foundation and Stellar fee oracle

[floxxih]

Summary

• Add a new Soroban isolated_lending contract package with a factory + pair architecture for isolated markets, including pair creation and isolated supply, withdraw, post_collateral, borrow, repay, and collateral-withdraw flow.
• Add a backend fee oracle service and expose GET /api/fees to estimate low/average/high transaction fees from recent Stellar ledger activity with in-memory caching.
• Update frontend transaction builders to query /api/fees and apply recommended dynamic base fees instead of hardcoded fees.

Notes

• This implements the foundational architecture and flow wiring for isolated lending and network-fee estimation.
• Tests were intentionally not added per request.

Closes #82
Closes #142

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