Fix display of negative fees when using native swap reservation

[savardd]

Description

Fix displaying of negative fees when doing native swap reservation.
This fix will be irrelevant once the PR#123 is merged on @btc-vision/transaction.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Performance improvement
• Refactoring (no functional changes)
• Documentation update
• CI/CD changes
• Dependencies update

Checklist

Build & Tests

• npm install completes without errors
• npm run build:chrome builds successfully
• Extension loads without errors in browser

Code Quality

• Code follows the project's coding standards
• No new TypeScript/ESLint warnings introduced
• Error handling is appropriate
• Console logs removed (except for error logging)

Documentation

• Code comments added for complex logic
• README updated (if applicable)

Security

• No sensitive data (keys, credentials) committed
• No new security vulnerabilities introduced
• Private keys are never logged or exposed
• User inputs are properly validated

OPWallet Specific

• Changes work across all supported browsers (Chrome, Firefox, Brave, Edge, Opera)
• Wallet state management is handled correctly
• Transaction signing follows security best practices
• RPC communication is secure
• Content script isolation is maintained
• Background/popup communication is secure

Testing

Browser Testing

• Chrome
• Firefox
• Brave
• Edge
• Opera

Screenshots

Related Issues

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By submitting this PR, I confirm that my contribution is made under the terms of the project's license.