Notify user during offline periods

In the absence of an internet connection, users must be informed that their exchange rates might not be up-to-date.

Additionally, users should be informed of the exact time when the rates were last updated. For this, we need to source such data from https://github.com/ARK-Builders/ark-exchange-rates.

No files or tests are named in the issue. Start by locating the offline exchange-rate flow and the UI that presents rates, then inspect the ark-exchange-rates repository for the available update-time data. Done means offline users are warned that rates may be outdated and can see when the rates were last updated.

Written by the indexing model from the issue text.