Debts page shows the last rate segment's APR, not today's #7

The Debts page shows a debt's APR from the last segment of its rate schedule, not the segment in force today. Apr() in src/Mossmint.Desktop/ViewModels/DebtsPage.cs (line 229) reads rates.Segments[^1].

So a card on a 0% intro APR shows the rate that starts after the promotion, and a loan with a scheduled rate change shows the future rate. The export uses rates.SegmentOn(today) (ProfileExport.cs:73), and so do the payoff and loan engines, so the page and the export can disagree for the same debt. The forecast itself isn't affected.

What to do

• Pass today from Card() into Apr() and use rates.SegmentOn(today).
• Decide what the page shows when no segment applies yet (SegmentOn returns null before the first one starts).
• Cards show Buckets[0]'s rate (line 170); check that's still the bucket to show.
• Consider showing the next change ("0.00% APR, 24.99% from 2027-03-01") when one is scheduled.

Done when

• The Debts page and the export show the same APR for the same debt and day
• A test covers a schedule with a future segment