## Why
Prod warning logs were flooded with `Exchange rate not found, returning
unconverted amount` (≈500 lines from a single dashboard load). The
`ExchangeRateService` was behaving correctly — the source currency was
the ISO 4217 placeholder **`XXX`** ("no currency"), which has no
exchange rate, so conversions silently fell back to an **unconverted
(wrong) amount**.
Root cause: when importing accounts from a banking connection (Enable
Banking can report `currency: "XXX"`), the code did
`$accountData['currency'] ?? 'EUR'`. The `??` only catches
`null`/missing, not the literal `"XXX"`, so the placeholder was
persisted as `currency_code`.
Prod data confirmed `XXX` is the **only** account currency not covered
by the rates table (342 currencies, incl. BTC/VES/exotics, are all
present): 18 accounts (16 from bank links, 2 manual) across 14 users,
plus **5 users** whose base currency had itself become `XXX` (a
first-account `XXX` propagates to the user via `syncFromFirstAccount`).
## What
1. **Fix the source** —
`AccountUserCurrencyService::resolveImportedCurrency()` resolves a
bank-reported currency to: bank value → user base currency → app default
(`cashier.currency`), treating `XXX`/empty/missing as "no currency".
Wired into both creation paths (`CreatesAccountsFromPending`,
`AccountMappingController`); also covers Interactive Brokers imports.
2. **Backfill** — migration fixes existing rows in order: `XXX` users →
app default first, then `XXX` accounts → their owner's currency.
## Tests
- `AccountUserCurrencyServiceTest` — resolver chain (valid / uppercasing
/ XXX·empty·null → user / both missing → default).
- `AccountMappingTest` — mapping flow falls back to the user currency
when the bank reports `XXX`.
- `BackfillXxxAccountCurrenciesTest` — migration resolves both XXX
owners and XXX accounts end-to-end.
23 tests green; Pint and Larastan clean.
bc57eae5c3