test: add missing UI test for popup error display

[cmuench]

This PR adds a test case to tests/popup.test.js that simulates a failure in the sendWebhook function (by mocking it to return a rejected promise). It then asserts that the UI updates correctly:

• The status message displays the error prefix and the error message.
• The status message has the error CSS class.
• The button text changes to the failure message.

This ensures that the error handling logic in popup.js is covered by tests, preventing regressions in error feedback to the user.

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PR created automatically by Jules for task 7454328377747214206 started by @cmuench

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