chat.js:
- reloadSession: during an active stream, replace the last built assistant
  message with the live streamingMsg instead of discarding it on text match.
  This fixes the root cause of invisible first messages and flickering tool
  cards when session_sync arrives mid-stream.
- buildMessageList: remove assistantCounter and use firstIdx as the message id
  to avoid duplicate Vue keys (e.g. user h_0 and assistant h_0).

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