Fix: Currency conversion math is inverted

[Copilot]

Fixed the currency conversion bug in app/components/Summary.tsx. The formula was inverted - the app was dividing when it should multiply, and multiplying when it should divide.

Changes Made

File: app/components/Summary.tsx (lines 21-26)

Changed the calculateTotal() function to use the correct conversion formula:

Before (incorrect):

const totalUSD = Object.entries(totals).reduce((acc, [currency, amount]) => {
  return acc + amount / (rates[currency] || 1)  // Wrong: dividing
}, 0)
return totalUSD * (rates[selectedCurrency] || 1)  // Wrong: multiplying

After (correct):

const totalUSD = Object.entries(totals).reduce((acc, [currency, amount]) => {
  return acc + amount * (rates[currency] || 1)  // Correct: multiplying
}, 0)
return totalUSD / (rates[selectedCurrency] || 1)  // Correct: dividing

How It Works

If the exchange rate API returns rates where rates[EUR] means "1 EUR = X USD":

• To convert FROM any currency TO USD: multiply by the rate
• To convert FROM USD TO selected currency: divide by the rate

Example with rates[EUR] = 1.09 (1 EUR = 1.09 USD):

• Convert 100 EUR to USD: 100 × 1.09 = 109 USD ✓
• Convert 109 USD to EUR: 109 ÷ 1.09 = 100 EUR ✓

Testing

• ✅ Build succeeds with no errors
• ✅ Application loads correctly
• ✅ Formula logic verified mathematically

Fixes #42

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Bug: Math is wrong</issue_title>
<issue_description>Sometimes when selecting different currencies the math is wrong, I believe I made a mistake when coding this app, find the math issue and fix it, address the problem at the root. The app is meant to be a local self hosted subscription tracker that allows you to input in multiple curencies as well as multiple services that will renew at different dates</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #42

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Project	Deployment	Preview	Comments	Updated (UTC)
subs	Ready	Preview	Comment	Oct 2, 2025 9:45am

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[ajnart]

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[ajnart]

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[ajnart]

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[Copilot]

@copilot continue on this

Fixed the currency conversion formula in commit 571f54e. The math was inverted - changed from divide-then-multiply to multiply-then-divide.

The fix ensures amounts are correctly converted to USD first (by multiplying with the rate), then converted to the selected display currency (by dividing by the rate).

Build verified successfully ✓

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