Add "Accept Suggestion" button to FeeOptimizer to apply the recommended fee in one click

[Kingsman-99]

Description

FeeOptimizer shows a recommended fee but requires the user to manually copy the value into the fee input. An "Accept Suggestion" button that applies the recommended fee directly to the transaction form removes friction and reduces input errors.

Acceptance Criteria

• An "Accept Suggestion" button is shown next to the fee recommendation
• Clicking it populates the transaction fee input with the suggested value
• The button is hidden when the current fee already matches the suggestion
• Existing tests continue to pass

Context

• Target file: src/components/FeeOptimizer.tsx