Fix delta loss during thinking-to-text transition and streaming orphaning ...

Backend:
- agent.py _consume_stream: replace elif with if so a single chunk can
  carry both the final thinking fragment AND the first text delta.
  Fixes the 'first token lost after tools' bug.

Frontend:
- chat.js reloadSession: bail out early when streaming.value is true.
  Replacing messages.value mid-stream orphans streamingMsg, breaking
  Vue reactivity for all subsequent deltas and tool cards.
- useWebSocket.js session_sync handler: add !chat.streaming guard
  before calling reloadSession.

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