webclient: remove 'Planning' label from wait indicator; add scroll-to-bottom button ...

- Waiting indicator now shows only a spinner (text was misleading — model
  doesn't always plan before responding)
- Add floating scroll-to-bottom button that appears when scrolled >200px
  from bottom, disappears when near bottom; smooth fade+slide transition;
  positioned bottom-right (under thumb on mobile)

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

webclient: message timestamps, confirm dialog, layout and UX fixes ...

- Add useTime.js: relative time labels ("just now", "5m ago", HH:MM) with auto-refresh every 30s
- Show message timestamps below user bubbles and assistant replies
- Show session last_active time in sidebar below preview
- Add ConfirmDialog.vue + useConfirm.js: kit-styled modal confirm, wired to delete in SessionItem
- SessionList: switch RecycleScroller → DynamicScroller to support variable item heights
- SessionItem: remove fixed 74px height; show action buttons always on touch devices (hover: none)
- MessageList: constrain content to max-width 920px centered (message-list-inner, input-row)
- MessageList: replace TransitionGroup with plain v-for; animate only new messages via .msg-enter CSS class, history loads silently without scroll animation

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>