docs(lethe-web-ui-foundation): import design handoff, add task file

Bring in the React prototype bundle from the claude-design handoff
(`docs/design_handoff_assistant_log/`) and the new task file
(`docs/tasks/lethe-web-ui-foundation.md`) covering the foundation slice
of the assistant-log web UI: scaffold, embed pipeline, shell, Home, and
Session view. Backend additions to `/api/v1/sessions` (summary,
turn_count, token totals, model) are part of this task; Projects, Stats,
Search, Health, and Settings are split into deferred sibling tasks.