- MessageList: pass isFlashing prop to UserMessage/AssistantMessage instead
  of applying msg-flash on wrapper div. scrollToMessage now uses rawIndices
  to map backend match_index to grouped message index.
- UserMessage: add isFlashing prop, apply msg-flash to msg-user-bubble
- AssistantMessage: add isFlashing prop, apply msg-flash to msg-assistant-content
- ChatStore: add rawIndices to built messages (user, summary, assistant)
- AppSidebar: move spinner into sessions-label, remove from search field
- SessionList: accept searchOpen prop, pass empty searchQuery when closed
  so highlights disappear after selecting a session
- Tests pass (51/51)

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