Fix link preview width when chat is in the sidebar

[nickvergessen]

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Copied from #7822

• Figure out a proper way to set the width
  • @marcoambrosini Maybe you can help here to share some insights on the reasoning for the flex values on the message, I've been able to align it with https://github.com/nextcloud/spreed/pull/7822/files#diff-5b376538abda3104135a2b8890732df2e72ba5533bae840015b0be25c5f33313R840 but not sure if that has side effects that I don't see

[nickvergessen]

Fixed by #7981