Usage: a daily heatmap for tokens and cost

[mrlufepines]

Problem

The Usage page is great for totals and for the chart over 24h to 90 days. What it doesn't show at a glance is the rhythm: which days I actually leaned on the agents, streaks, quiet weeks, and how that lines up with limits. That's the view I keep wanting, a grid of small squares per day like GitHub's contribution graph.

Proposal

A heatmap on the Usage page, read-only and additive:

• one square per day, colored by tokens or cost, following the current Cost / Tokens view
• respects the current filters (provider, model, environment)
• hover or tap shows the day's total and its top models
• built from the daily buckets usageSummary already returns, so no contract or server change and no scanning beyond the current 90-day range

Web first, then mobile with the same data.

Notes

• Per-account and per-environment breakdowns are being discussed in Web: break Usage down by project, thread, account and environment #16722 and Usage page: break down spend by provider account (instance) #27154. This doesn't depend on them, but would pick them up through the filters.
• Happy to build it if the direction makes sense.