feat: metro-map workflow with presets, color picker, agent assignment

Rewrite WorkflowPage as metro-map style inspired by Incident &
Problem Solving methodology:
- 3 workflow presets: Incident Solving, Problem Solving, Change Mgmt
- Metro station circle nodes with thick colored edge lines
- Edge color inherited from outgoing node
- Click node → dialog with color picker (8 colors) and agent selector
  (10 agent presets)
- Agent indicator dot on nodes with assigned agents
- Color legend auto-generated from used colors
- 12 Playwright E2E tests covering presets, node config, animation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: abossard