Blocker and High priority fixes

[jenbreese]

Summary

Moves the "balance grows if left unpaid" line into the "When you borrow" explanation where it belongs, and switches the calculation to use the fractional periods value the user enters rather than a rounded whole number. Also removes the stray "Rounded to the nearest whole period" info message that was added to the errors doc by mistake.

Changes

• Appended "This shows how the balance grows if left unpaid." to the borrowing explanation copy so it renders as constant context, not a result-dependent line.
• Removed the borrowing helper block from the results panel. It was gated on !hasError, which made it appear above the results as soon as a valid calculation ran.
• Removed rounding in the calculation useMemo. Math.pow now receives the fractional periods directly, so 2.5 periods computes as 2.5, not 3.
• Removed the periodsInfo state and its "Rounded to the nearest whole period for calculation" message entirely (state declaration, reset handler, both change-handler branches, and the message paragraph fallback).

Testing

• $1 / 8% / 2.5 periods / annually now returns a final amount of $1.21 as expected.
• Verified the borrowing line no longer appears above the results in error or valid states, and shows only in the "When you borrow" section.
• Confirmed zero-period warning, range errors, and whole-number results are unchanged.

[majimmy88]

LGTM