Show projected end-of-period usage

**Goal.** See where usage is heading before the period ends, while there is still time to act. Today the page only shows what has already happened.

**Build.** One calculation, three places:

- a **Projected end-of-period** tile
- a **dashed line** on the chart, from today to the period end
- a **sentence under the meter**, like "On track to use ~1.9M (94% of your limit) by 17 Aug", or that it lands over the limit

**Hide it early in the period**, where an average means very little. Pick the point: a number of days, or a share of the period.

**Never on a rolling window.** Free plans have no period end to project to, and their total can fall as old days drop out, so a projection there is misleading rather than merely early.

**Call it an estimate.** A straight-line average of usage so far, not a forecast.

**Needs #8257.** The projection divides by the period length, which the API does not return today.

**Done when**

- the projection appears in all three places once there is enough of the period to mean anything, and in none of them before
- a projection that lands over the limit is obvious without working it out