[Bug] `responseBody` used before declaration in `tipService.ts` causes ReferenceError on API errors

[RUKAYAT-CODER]

Overview

In src/services/tipService.ts around line 29, responseBody is referenced in the error-path branch before it is ever assigned. The variable holding the parsed JSON is stored under a different name, so when the API returns a non-2xx status the service throws ReferenceError: responseBody is not defined instead of a meaningful error message.

Specifications

Features:

• Tip failures surface a meaningful error message to the caller rather than crashing

Tasks:

• Replace responseBody.error with the correctly named variable (e.g. response.error after awaiting res.json())
• Add a null-safe fallback: typeof errorBody?.error === 'string' ? errorBody.error : 'Unable to send tip.'
• Write a unit test that mocks a non-OK API response and asserts the error message is returned cleanly

Impacted Files:

• src/services/tipService.ts

Acceptance Criteria

• No ReferenceError is thrown when the tips API returns a non-2xx response
• The caller receives the server error string or the fallback message
• Existing tip success tests continue to pass