Restore current and past usage estimates

[thrr87]

Closes #31

What changed

• keep the current estimate visible when fresh account points show a useful direction
• use complete account history first, then a dense reset-aware account-history fallback for the chart
• keep low-confidence chart lines out of guidance, status, runway, and Insights
• label the past line as Account history

Checks

• 424 Swift tests passed
• full-screen UI check passed with Current estimate and Past estimate visible