fix(exchange-rate): return aValue fallback when exchange rate data is missing

Fixes #6482

toCurrencyAtDate() previously returned undefined when no historical
exchange rate data was available for the requested date. This undefined
propagated into activity fields like feeInBaseCurrency and
valueInBaseCurrency, causing [big.js] Invalid number crashes on the
Overview, Portfolio, and Holdings pages.

Fix: log a warning and return the original aValue (1:1 fallback) instead
of undefined, preventing the crash while gracefully degrading.

Signed-off-by: Venkateswarlu Boggavarapu <mailtoboggavarapu@gmail.com>

apps/api/src/services/exchange-rate-data/exchange-rate-data.service.ts

@@ -349,7 +349,11 @@ export class ExchangeRateDataService {
       'ExchangeRateDataService'
     );
 
-    return undefined;
+    Logger.warn(
+      `No exchange rate data found for ${aFromCurrency} -> ${aToCurrency} on ${aDate.toISOString()}, returning original value as fallback`,
+      'ExchangeRateDataService'
+    );
+    return aValue;
   }
 
   private async getExchangeRates({