Add 8px below the activity summary to separate it from new conversation. Remove redundant conversation-row arrows while keeping project submenu arrows, and update the UI documentation.
Validation: 11 ProjectActivity tests and typecheck passed. Lint was interrupted before completion.
Release note: 调整项目活动摘要与新建会话的间距，移除会话列表末尾的多余箭头，保留项目子菜单指示。
Replace the production activity dialog with anchored project and conversation menus, preserving exact conversation navigation and adding keyboard and narrow-window coverage. Include the interactive demo and design documentation.
The demo hides idle activity and removes conversation arrows; production still retains those two elements. Validation: typecheck, lint and 11 ProjectActivity tests passed. The full suite timed out after 240 seconds with an agent-package offline inventory test failure.
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