Fix scroll-to-bottom after page reload ...

Watch chat.loading instead of currentId for post-load scroll.
nextTick + rAF ensures browser layout is settled before scrolling.

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

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>

Migrate to Vue webclient; rename old client to old_webclient ...

- client/ → old_webclient/ (vanilla JS client preserved as reference)
- webclient/ — new Vue 3 + Pinia webclient (source + dist build)
- vite.config.js: outDir changed to webclient/dist/
- main.py: serve /assets and /images from webclient/dist/,
  index.html from webclient/dist/index.html
- .gitignore: exclude webclient/node_modules/, include webclient/dist/

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