accurate fee estimation

[Jumongweb]

Closes #638

Summary

• Replaces static fee multipliers with Horizon /fee_stats percentiles.
• Simulates Soroban transactions using the existing RPC client.
• Displays CPU instructions, read/write bytes, resource fee, base-fee range, total XLM, and stroops.
• Adds a conservative fallback with a visible warning when simulation is unavailable.
• Re-estimates whenever transaction inputs change.
• Preserves the existing FeeEstimator callback contract.
• Adds unit and component coverage for fee calculation, rendering, fallback behavior, and reactive updates.

Testing

• Focused fee and rendering tests: 24 passed.
• git diff --check: passed.
• npm run type-check: currently blocked by pre-existing unrelated TypeScript errors across the repository.
• Full npm test: currently blocked by pre-existing unrelated wallet, escrow, CSV, snapshot, and missing-module failures. The new and modified fee-related suites pass.

[github-actions]

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