Feat/fee estimation service

[Valreb001]

Closes #400

What was re-implemented in feeEstimation.ts:

• Horizon fee_stats endpoint — queries real p10/p50/p90 fee percentiles instead of hardcoded
  BASE_FEE
• TTL cache — 10-second cache to avoid hammering the API on every render
• Surge pricing — applies a 1.5× multiplier when ledger capacity usage exceeds 80%
• Fee tiers — min (p10), recommended (p50), max (p90), all in XLM
• Graceful fallback — returns safe BASE_FEE-based values if Horizon is unreachable
• Fully typed with TypeScript, singleton export, and a clearCache() method for testing

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