Add inline probe highlighting and threshold slider

- Render probe scores as inline background tints over the assistant
  text instead of a separate heat strip
- Add per-user highlight threshold slider (localStorage-persisted) so
  low-confidence tokens can be hidden from the chat view
- Switch probe scorer to non-streaming chat completions; reads scores
  from top-level response.scores, drops the -analyze model variant
- Emit content_done SSE event so the typing indicator drops as soon
  as Claude finishes, while probe scores keep streaming in

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>