Restore Current and Past estimates in the usage chart

[thrr87]

The Current window chart can hide both forecast lines when coverage is low. The old chart still showed where the current pace and past pace would end.

Restore:

• Current estimate when fresh account readings provide a measurable pace
• Past estimate from complete account history, or from dense reset-aware history for the chart only
• source, coverage, and confidence labels

Do not let the chart-only fallback change Suggested pace, Runway, status, or Insights.

Acceptance:

• Current and Past start at Now and end at reset or 0%
• failed or ended account reads do not show a forecast
• sparse history does not create a Past estimate
• tests cover low coverage and dense partial history
• full-screen QA confirms both lines and no debug text

[thrr87]

Shipped in #32.

Verified with 424 passing Swift tests, green Swift tests and CodeQL in CI, and a full-screen UI check showing both Current estimate and Past estimate. Low-confidence chart lines remain chart-only and do not change guidance, runway, status, or Insights.