- session open: centered spinner in MessageList while chat.loading
- image paste/drop: imagesProcessing counter (chat store, shared);
  FilePreviewStrip shows spinner placeholder tiles where the resized
  previews will land
- file upload: indeterminate sliding bar instead of the fake static 60%
- send → run start gap: "Sending…" hint with spinner, cleared on
  stream_start / message_queued / stream end / error / session switch
- content-card images: skeleton with spinner until @load (reset on src
  change); wrapper keeps min-height while the img is empty

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