## Why
### Problem
Users with accounts in multiple currencies see raw balances on the net
worth chart without any conversion. A user with a USD-denominated
profile who also holds EUR and GBP accounts sees misleading totals — the
chart simply sums cents across currencies as if they were the same unit.
## What
### Changes
- **`ExchangeRateService`** — New service that wraps
`CurrencyConversionService` with DB-level caching. Checks the
`exchange_rates` table first, falls back to the external CDN API on
cache miss, and stores the full rate blob for future lookups.
- **`exchange_rates` migration + model** — New table with
`(base_currency, date)` unique constraint storing JSON rate blobs.
Includes factory.
- **`CurrencyConversionService`** — Made `getRatesForCurrency()` public
so `ExchangeRateService` can delegate to it.
- **`DashboardAnalyticsController`** — `netWorthEvolution` and
`netWorthDailyEvolution` endpoints now convert every account balance
from `account.currency_code` to `user.currency_code`. Response includes
`currency_code` field and `_original` data for foreign-currency
accounts.
- **Future date capping** — `ExchangeRateService::getRates()` caps any
future date to today, since the currency API only serves historical
rates. Prevents 500 errors when the chart requests end-of-month rates
for the current (incomplete) month.
- **Frontend charts** — Simplified to single-currency display. Removed
multi-currency `TotalDisplay` component; the chart header and tooltip
now show amounts in the user's currency. Foreign-currency accounts show
their original amount inline in the tooltip (e.g., "€2,000.00 →
$2,222.22").
- **`valueFormatter` type fix** — Changed return type from `string` to
`React.ReactNode` in chart component interfaces to match actual usage
with `AmountDisplay`.
## Screenshot
<img width="1266" height="649" alt="image"
src="https://github.com/user-attachments/assets/6f661840-3f65-49fe-9948-d4cfd6c6ef0e"
/>
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