Display the billing period dates on the dashboard

**Goal.** Someone can see which billing period they are in, when their allowance resets, and how long is left.

Three things on the usage page describe the period itself, and none can be read from the API today:

- the period's date range
- when the allowance resets
- days remaining

Needs #8257. Free plans have no period, so they show nothing in their place, which the page already handles.

**Done when** someone on a paid plan can see all three, and someone on a rolling window sees none of them.