[ENG-3436] chore: Make some code refactoring for RBF in the extension

[dhriaznov]

🔘 PR Type

What kind of change does this PR introduce?

• [x] Refactoring (no functional changes, no API changes)

📜 Background

After the RBF UI & logic were added to the web-extension, we also developed it for the mobile app. And some things could be improved in its code like constants, typing, etc. (kudos to @fedeerbes for the mobile app PR review)

This PR is made to address those things for the web extension as well.

Issue Link: #[ENG-3436] Context Link (if applicable):

🔄 Changes

• Moved Speed Up tx custom fee component to a separate folder and created a separate file for its styles
• Removed the initialFeeRate property of the custom fee component that had the same value as minimumRbfFee
• Fixed typing for the Speed Up tx screen
• Moved the handleKeyDownFeeRateInput function to helpers to reuse it
• Added EMPTY_LABEL constant
• Used the FiatAmountText component instead of duplicating the getFiatAmountString function
• Moved the copy related to estimated RBF completion time to translations

Impact:

• This PR makes the RBF code more clean and robust, and shouldn't break any logic

🖼 Screenshot / 📹 Video

Include screenshots or a video demonstrating the changes. This is especially helpful for UI changes.

✅ Review checklist

Please ensure the following are true before merging:

• [ ] Code Style is consistent with the project guidelines.
• [ ] Code is readable and well-commented.
• [ ] No unnecessary or debugging code has been added.
• [ ] Security considerations have been taken into account.
• [ ] The change has been manually tested and works as expected.
• [ ] Breaking changes and their impacts have been considered and documented.
• [ ] Code does not introduce new technical debt or issues.

[dhriaznov]

Thanks for the review @teebszet! I updated the PR, and also included the fix from https://github.com/secretkeylabs/xverse-web-extension/pull/713 you created since it's related and would be better to test those 2 PRs together. Could you approve it again pls? 🙂

[dhriaznov]

It would make sense to test and merge this PR together with https://github.com/secretkeylabs/xverse-web-extension/pull/727 in the next release (0.28.0)

[github-actions[bot]]

Test with build here: https://github.com/secretkeylabs/xverse-web-extension/actions/runs/7357186386

[DuskaT021]

@dhriaznov this can be merged, tested per agreement