Paginate session list loading

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WelcomeScreen polish, root-path fix, docs update ...

- Move mobile sidebar button to top-left corner (no header bar backdrop)
- Show WelcomeScreen on / with no hash instead of auto-loading first session
- Docs: document Ollama multi-server fallback, model priority lists, OLLAMA_BACKENDS_FILE

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

Fix WelcomeScreen: add with-icon, mobile sidebar toggle ...

- Add with-icon class to Start button
- Add mobile-only header with sidebar toggle (reuses chat-header +
  btn-sidebar-toggle styles, hidden on desktop via existing CSS)
- Wire toggle-sidebar event through App.vue

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

Go to WelcomeScreen when active session is deleted ...

Previously deleting the active session would load the first session from
the list. Now it calls clearSession() which resets currentId to null,
and showWelcome now triggers on currentId === null (not just empty list).

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

Webclient UI improvements + backend fixes ...

Webclient:
- Draft persistence across page refreshes (localStorage, reactive watch)
- Image lightbox modal using UI kit classes on thumbnail click
- Copy button on user and assistant messages
- Selection reply toolbar: select assistant text → quote inserted into input
- User message rendering: proper HTML escaping, styled blockquote for > replies
- Markdown table fix: preprocessor to inject missing separator rows
- Planning status labels (rebuild dist)

Backend:
- Developer profile: enable subagent delegation, increase max_iterations to 35
- share_file: updated description + manual with absolute path requirement and URL sharing
- persona.txt: instructions for quote replies and GFM table format

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>

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>