fix: handle tip service API errors safely

[pharwasz]

Fixed tip service error handling for non-2xx API responses.

• Replaced the undefined responseBody error path with null-safe error handling.
• Added a fallback message: Unable to send tip.
• Added a regression test covering non-OK API responses.
• Verified the existing success path continues to pass.

Related Issue

Closes #703

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Checklist

• Code follows project style guidelines
• Self-review completed
• No console errors
• Uses Lucide icons consistently
• Responsive design implemented
• Starknet best practices followed

Testing

• pnpm exec vitest run src/services/__tests__/tipService.test.ts
• 1 test file passed
• 4 tests passed

Note: Prettier reported a formatting issue in tipService.ts; no functional test failures were observed.

[drips-wave]

@pharwasz Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[RUKAYAT-CODER]

Thank you for contributing to the project.