Usage: finer buckets for the 7 days window

[spiky02plateau]

On the Usage page, the 7 days window is the coarsest view in practice.
Past 24h draws 24 hourly points and 30 days draws 30 daily points, but 7 days draws only 7 daily points, so the smoothed curve hides when work actually happened.

7 days today	7 days with hourly buckets

I'd like direction on whether the 7 days view should use finer buckets, and if so which shape you'd accept:

1. Hourly (168 points). Reuses the existing hourly aggregation; the server's hourly window cap goes from 24h to 7 days. This is what fix(web): 7-day usage chart shows hourly variation #18177 does today.
2. A coarser fixed bucket such as 3h or 6h (56 or 28 points). Smoother chart, but needs a new resolution in the contract.
3. Keep daily. If 7 days is meant as a day level summary, I'll close the PR.

Things any option above daily has to handle, which I'd include in scope:

• Mobile uses the same days === 1 rule, so it should change in the same PR to keep clients consistent.
• An older server rejects an hourly window above 24h with invalidWindow. The client can fall back to daily for that environment instead of showing an error.

Draft implementation: #18177.