Fix fee estimator

[T-kesh]

feat: add robust FeeEstimator with concurrency guard, loading spinner, and retry button

• Implements a concurrency guard using useRef to prevent overlapping load calls.
• Adds safe client checks via hasClient().
• Shows a non‑blocking loading spinner while preserving the last known fee.
• Displays error messages with a retry button (accessible via ARIA label).
• Polls fee estimates every 10 seconds with exhaustive dependency handling.
• Includes live region for screen‑reader announcements.
• Updated component tests to cover new UI and concurrency behavior.
  closes fix(FeeEstimator): concurrent load race condition, no loading skeleton, no retry button after error #13

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