fix(web): Handle image loading failures in laptop screen generator #95

Problem

The laptop screen generator handles successful image loading through img.onload, but does not handle img.onerror.

When a user selects a corrupted or unsupported image, the UI does not report the loading failure. If a valid image was previously loaded, it remains active, potentially causing the user to download a PDF containing the wrong image.

Location

web/js/screens.js — loadImage()

Introduced in PR #88.

Expected behavior

• Display a clear error message when an image fails to load.
• Avoid silently using a previously loaded image.
• Keep the UI state consistent with the selected file.
• Allow users to retry with another image.

Acceptance criteria

• Handle image decoding/loading failures using img.onerror.
• Display an actionable error message.
• Ensure the preview and PDF download cannot silently use the wrong image.
• Verify behavior when loading an invalid image after a valid one.