Added errors and standards to interest cal

[jenbreese]

READY FOR REVIEW

Summary

Notes: I added the errors in the error doc. I also updated some of the UI to match the new styles we are doing. I brought up some of the consistency issues.

Interest Calculator -- validation, formatting, and accessibility

Validation

• Added empty field error on Initial amount: blocks calc, "Please enter an initial amount."
• Added range error on Initial amount (outside $0-$100,000,000): blocks calc, "Enter an amount between $0 and $100,000,000."
• Added empty field error on Interest rate: blocks calc, "Please enter an interest rate."
• Added range error on Interest rate (outside 0%-1,000%): blocks calc, "Enter a rate between 0% and 1,000%."
• Added warning on Interest rate at 0%: "At 0%, no interest is earned or charged — final amount equals initial amount."
• Added empty field error on Compounding periods: blocks calc, "Please enter the number of compounding periods."
• Added range error on Compounding periods: dynamic message based on selected frequency, matching PV calculator pattern (e.g. "Enter a number of months between 0 and 1,200. (1,200 months = 100 years with monthly compounding).")
• Added warning on Compounding periods at 0: "0 periods means no time passes — final amount will equal the initial amount."
• Added info state on Compounding periods for decimals: "Rounded to the nearest whole period for calculation."
• Compounding frequency change revalidates periods against the new frequency's max
• Results panel shows "—" in all three output rows when any field has an error or is empty
• "Fix the highlighted field to see results." message shown above results when errors are present

Formatting

• Initial amount input now displays comma-formatted values using the raw/display pattern from the inflation calculator
• Stepper buttons update both raw and display state and revalidate on click
• Results panel uses formatCurrency via Intl.NumberFormat for consistent USD output across all three rows

Refactoring

• frequencyMap expanded to include periodLabel per frequency, used in dynamic error messages
• freqAdjective map added for error message copy
• buildPeriodsRangeError helper lifted to module level, matching PV calculator pattern
• Calculation moved from useEffect to useMemo with 300ms debounced inputs
• All field onChange/onBlur handlers extracted to named functions

Accessibility

• aria-pressed added to Saving/Borrowing mode toggle buttons
• aria-invalid and aria-describedby added to all three inputs
• Error/warning <p> elements always rendered, hidden with sr-only when inactive so aria-describedby always resolves
• role="alert" on error messages, omitted on warnings and info states
• Results panel wrapped in aria-live="polite" aria-atomic="true"
• aria-label on all stepper buttons