### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### The Feature

### The Feature

Allow selecting multiple API keys belonging to the same user and visualizing their daily usage (spend, tokens, requests) as comparable time series on a single chart in the Usage tab.

### Current state

The backend already returns everything needed: `GET /user/daily/activity?start_date=...&end_date=...` includes `breakdown.api_keys` per day with spend / prompt_tokens / completion_tokens / api_requests. This is purely a UI gap.

### Proposal

On the Usage tab (when scoped to a single user):

- A multi-select listing that user's keys by key_aliases - A line chart with one series per selected key, x = date, y = spend, with a toggle for tokens / requests - Keep the existing aggregated view as default; the multi-key comparison is an opt-in mode

### Motivation, pitch

In our deployment, individual users own many keys, typically one per tool/integration (Claude Code, Zed, internal scripts, CI jobs, etc.). The current Usage tab shows a breakdown by key, but there's no way to chart several keys of a single user as overlaid series on the …