Steadier meeting recaps and screen shares

FixedScreen-share screenshots show up right away. A screenshot Tana captures during a screen share no longer briefly fails to load for other participants in the meeting. The latest screenshot also wins display order, even if an older one finishes uploading last.
FixedMeeting recaps no longer wait forever on a missing cover image. When the cover illustration for a meeting recap cannot be saved, the recap is written without one instead of leaving a permanent loading block in its place. Re-summarizing the meeting brings back the missing image.