Add human-readable fee breakdown response to /fee-estimate

[Sulex45]

Description:
The current /fee-estimate response returns numbers but doesn't explain what they mean to a developer who is new to Stellar. Extend the response to include a plain-English recommendation and context about when to use each fee tier.
Acceptance Criteria:

Adds recommendation field to response explaining which tier to use and why
Adds context field explaining what stroops are in plain English
Adds networkCongestion field: "low", "medium", or "high" based on ledgerCapacityUsage
networkCongestion thresholds: low < 0.5, medium < 0.75, high >= 0.75
No breaking changes to existing response fields
Test added

[martinshub-tech]

@martinshub-tech has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello. Please assign this issue to me. I checked it out and I'm confident I can solve it.
I am a full stack developer.

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[drips-wave]

Congratulations, @martinshub-tech! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @martinshub-tech: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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