Lf improve configure fees design

[leofelix077]

Implements #413.

Reworks the transaction‑settings fee UI from a single free‑text input into a Low / Med / High / Custom priority selector, and makes the network fee data consistent and stable for the duration of a single Send / Swap / Send‑Collectible flow (no mid‑flow flicker). Presets are derived from Horizon feeStats, and the default tier follows current network congestion.

Builds on the existing FeeBreakdownBottomSheet (inclusion / resource / total) shipped earlier for this ticket.

Currently uses the API response of p10/p50 and p90 for low/med/high, instead of the mode of the recent values for the derivation

Checklist

PR structure

• This PR does not mix refactoring changes with feature changes (break it down into smaller PRs if not).
• This PR has reasonably narrow scope (break it down into smaller PRs if not).
• This PR includes relevant before and after screenshots/videos highlighting these changes.
• I took the time to review my own PR.

Testing

• These changes have been tested and confirmed to work as intended on Android.
• These changes have been tested and confirmed to work as intended on iOS.
• These changes have been tested and confirmed to work as intended on small iOS screens.
• These changes have been tested and confirmed to work as intended on small Android screens.
• I have tried to break these changes while extensively testing them.
• [ x This PR adds tests for the new functionality or fixes.

Release

• This is not a breaking change.
• This PR updates existing JSDocs when applicable.
• This PR adds JSDocs to new functionalities.
• I've checked with the product team if we should add metrics to these changes.
• I've shared relevant before and after screenshots/videos highlighting these changes with the design team and they've approved the changes.

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[github-actions]

iOS Simulator preview build is ready: https://github.com/stellar/freighter-mobile/releases/tag/untagged-2d06b96c5664f3c044e5 (SDF collaborators only — install instructions in the release description)