New IFxConverter.ConvertToChf(amount, fromCurrency) and FxConverter
wrap the vendor IFxService and the FixerIo API key so callers stop
threading the key through every conversion. ViewService drops
IFxService and ISettingsService from its constructor and takes
IFxConverter instead; GetAccountViewList and GetBalanceSheetView no
longer read settingsService.GetSettings() per method.
FxConverterTestShould locks in the target-currency + API-key routing.
40 tests total, all passing.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>