fixed: Video preview turns black when you attach another video or image into a yet unsent message

[Eichhorn10]

First time contributor checklist:

• I have read the README and Contributor Guidelines
• I have signed the Contributor Licence Agreement

Contributor checklist:

• My contribution is not related to translations.
• My commits are in nice logical chunks with good commit messages
• My changes are rebased on the latest main branch
• A pnpm run ready run passes successfully (more about tests here)
• My changes are ready to be shipped to users

Description

This fix ensures that draft attachments retain their nested screenshot data and legacy screenshotPath after being restored. Users can therefore continue to see and use draft attachments with their associated screenshots correctly.

Testing

• Manually tested adding multiple videos, removing them again, and sending them successfully
• Tested on CachyOS (Arch Linux) with KDE Plasma 6.7.4 and no other devices
• Added unit tests covering:
  • attachments with nested screenshot metadata
  • attachments with legacy screenshotPath
  • image attachments without screenshots
  • video attachments without screenshots

Fixes #7222