fix(ambient-ui): chat dark mode, markdown rendering, empty bubble filter

- Replace hardcoded hex colors with semantic Tailwind tokens for dark mode
- Add react-markdown + remark-gfm for assistant message rendering
- Add @tailwindcss/typography for prose styling
- Fix tool payload parsing to handle operational event format
- Filter out empty assistant message bubbles
- Strip wrapping quotes from tool result display
- Hide empty arguments section when tool has no args
- Spec: add Persistent Chat Sidebar requirement, update Chat tab scenario

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jsell-rh