Update exchange rates

Rate shown is 1 unit of the foreign currency in base currency, matching how Manager displays exchange rates. "Old rate" is the rate Manager would currently apply as at the chosen date (the latest rate recorded on or before it). If a rate is already recorded for that exact date, it's shown struck through immediately before the new rate, since updating will replace it. Frankfurter falls back to the latest prior business day when the chosen date has no published rate (e.g. weekends), which is noted per row. Rates are fetched automatically whenever the date changes. Updating requires an access token (Settings → Access Tokens) configured in CONFIG.accessToken at the top of this file — without one the tool still works for reviewing rates, but the update button stays disabled.

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