ThinkingCard: add chevron, open by default; plan loading indicator ...

- ThinkingCard: add ph-caret-down chevron (matches plan/tool card style),
  open by default (removed auto-close on thinking_end)
- AssistantMessage: show "Planning…" spinner when streaming but no content
  yet — gives feedback during the two-LLM-call planning phase
- app.scss: thinking-chevron styles, planning-indicator styles

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>