* feat(dashboard): add traffic flow sankey chart
Add dashboard flow APIs and a Sankey-based flow view with user, optional API key, model, and channel layers.\n\nReuse the dashboard VChart palette, add precise link/node tooltips and interactions, and cover filtering, layer ordering, color stability, and error states with tests.
* feat: build flow chart from quota data
---------
Co-authored-by: CaIon <i@caion.me>
