Added in the errors from the documentation

[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.

Mortgage Calculator -- validation, error states, and conversion fixes

Validation -- shared fields

• Interest rate: empty error ("Please enter an interest rate.") and range error for values outside 0-20% ("Please enter an interest rate between 0 and 20%.")
• Down payment %: range error for values outside 0-99.9% ("Enter a percentage between 0 - 99.9%.")
• Property tax %: range error for values outside 0-10% ("Enter a property tax rate between 0 and 10%. Rates beyond this are unusual.")
• Property tax $: relative range error when amount exceeds 10% of home price
• Homeowners insurance %: range error for values outside 0-10%
• Homeowners insurance $: relative range error when amount exceeds 10% of home price
• HOA dues: range error for values outside $0-$20,000 (cap lowered from $100,000)
• Property tax and insurance % defaults fixed from locale-formatted 1,25 / 0,35 to US decimal 1.25 / 0.35

Validation -- Tab 1 (Home you can afford)

• Monthly payment: empty error and hard range cap of $1-$1,000,000
• Down payment $: absolute range cap of $0-$1,000,000,000
• Output backstop: if back-calculated home price exceeds display bounds, shows "That payment is too high to calculate. Try a lower amount to see your estimate."

Validation -- Tab 2 (Monthly payment)

• Home price: empty error and range error for values outside $1-$1,000,000,000
• Down payment $: negative error and exceeds-home-price error ("Your down payment can't exceed the home price. Try lowering it.")

Math fixes

• Added loanAmount <= 0 guard in Tab 2 to prevent a $0 monthly payment flash when down payment equals or exceeds home price before the error state has propagated
• Removed state-setting side effects from calculateMortgage -- the function is now purely computational with no display updates

Dollar/percent conversion fixes

• Tab 1 dollar mode: the percent display is now a derived value computed from downPaymentAmount / calculatedHomePrice and passed as read-only display only -- it no longer feeds back into the calculation, eliminating the circular dependency
• Tab 2 dollar mode: percent syncs correctly in handleDownPaymentAmountChange and in the home price onChange handler where the price is immediately available
• Mode switch (percent to dollar): converts using (downPaymentPercent / 100) * price
• Mode switch (dollar to percent): converts using downPaymentAmount / price
• Tab 1 with no calculated price yet: dollar input shows blank on mode switch rather than a stale or incorrect value
• Property tax and insurance conversions use .toFixed(4) to avoid rounding drift through repeated mode switches

Architecture

• Page-level validation banner removed; all errors are inline next to their fields per spec cross-tab note Adding .github templates. #3
• All field components (InterestRateField, LoanTermField, DownPaymentField, PropertyTaxField, HomeInsuranceField, HoaField, ResultsPanel, ResultRow, FieldMessage) lifted to module level with explicit TypeScript prop interfaces, fixing a focus-loss bug where inner components were remounting on every render
• All state and handlers live in MortgageCalculator and are passed as props
• FieldMessage always renders (hidden with sr-only when inactive) so aria-describedby always resolves to a valid element