Fixes a regression where image attachments in user chat messages were
rendered twice, once inside the bubble container and once outside it.
- **ConversationTimeline.tsx**: Remove 43 duplicate lines (outer image
block + second fade overlay) from the `ChatMessageItem` user-message
branch.
- **ConversationTimeline.stories.tsx** (new): Add focused stories for
`ConversationTimeline` with `play` function assertions on image
thumbnail counts to guard against this class of regression.
Mathias Fredriksson