Show projected end-of-period usage #8188

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, two places:

• 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 both places once there is enough of the period to mean anything, and in neither before
• a projection that lands over the limit is obvious without working it out