feat(phase5): implement Vue3 portal foundation with chat interface and routing (U13a)
- Add Portal API routes: chat, stream, capabilities, conversations, WebSocket
- Add ConversationStore for in-memory conversation management
- Add CAPABILITY_CATEGORIES mapping for 8 capability types
- Create Vue3 SPA with TypeScript, Pinia, Vue Router, Ant Design Vue
- Implement ChatView with message bubbles, input, sidebar, WebSocket support
- Add side navigation skeleton for all 8 capability sections
- Add placeholder views for workflow, knowledge, skills, terminal, etc.
- 31 backend tests passing
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