webclient: cap thinking body at 800px, smooth details close, Navi-viewed images grouped in artifacts ...

- thinking-body: max-height 800px + inner scroll so long reasoning
  doesn't grow the whole chat
- content-card: body stays mounted after first open (no v-show teardown
  at toggle time) — ::details-content now has real content to shrink,
  so collapsing animates smoothly instead of snapping to 0 in a frame
- synthetic '[Image loaded via ...]' messages from vision tools are no
  longer rendered as user bubbles; their images are collected (live from
  tool_call metadata, and from history) into a viewed-images group at
  the top of the artifacts tab, thumbnails open the lightbox