* 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>
- Add GET /api/data/users endpoint for user-grouped quota data (admin only)
- Add user consumption ranking (horizontal bar, top 10) and user consumption
trend (area chart) tabs visible only to admin users
- Fix mislabeled "消耗趋势" tab to "调用趋势" (shows call counts, not quota)
- Add processUserData helper for user ranking and trend data extraction
- Add i18n keys for new tabs across all 7 locales
