feat(ui): port agent activation badge from Windows client for AI thinking visualization

- Add AgentActivationBadge component with animated role cards showing
  active agents (诊断师/策略师/内容师/技术顾问) during streaming
- Add RoleTraceIcon for role icons in trace display lines
- Add skill-to-role mapping (SKILL_ROLE_MAP) in messageTraceDisplay
- Add resolveRoleFromSkill with token-boundary matching to prevent
  false positives
- Add getActiveRoleFromTraces, getActiveRoleIdFromMessages utilities
- Wire activeRoleId in App.tsx (resolves existing TODO)
- Add agent card CSS with enter/exit animations, accent pulse, and
  working dot animations
- Add reduced-motion and responsive (720px) CSS overrides

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>