fixup for 6/25 changes

[jenbreese]

READY FOR REVIEW

Time Value of Money Calculator: Bug fixes and code quality improvements

Bug fixes

• RATE with mismatched frequencies: Fixed incorrect effective annual rate being returned when compounding and payment frequencies differ. The solver now correctly returns the nominal annual rate by reversing the cross-frequency conversion on the solved per-period rate.
• Validation error message formatting: Fixed negative currency constraint messages displaying as -$-1,000,000,000. Present value, future value, and payment error messages now correctly format as -$1,000,000,000.
• Payment timing reset on tab switch: Payment timing no longer persists as "Beginning" when switching between solve-for tabs. It now resets to "End" along with the other fields.

Code quality

• HowToUseInfoBox: Inlined the component JSX directly into the render output. Previously defined as a function inside Page(), it was recreated on every render, causing React to see a new component type each render and fully remount it, resetting any internal state.

[majimmy88]

Not something we need fixed here, but I'm noticing a lot of code formatting with no actual changes. We should implement a unified code formatter so the code reviews are smaller.