fix(web): Prevent Object URL memory leaks in laptop screen generator #96

Problem

The laptop screen generator creates a new Object URL using URL.createObjectURL(file) whenever a user selects an image, but never releases it with URL.revokeObjectURL().

Repeatedly loading large images can retain unnecessary memory for the lifetime of the page.

Location

web/js/screens.js — loadImage()

Introduced in PR #88.

Expected behavior

Object URLs should be released once they are no longer needed, without affecting the currently displayed image or PDF generation.

Acceptance criteria

• Revoke Object URLs after successful image loading when safe.
• Revoke Object URLs when image loading fails.
• Avoid retaining URLs from previously selected images.
• Verify that repeated image replacements do not accumulate unused Object URLs.
• Ensure image previews and PDF downloads continue working correctly.