Implement updated chronik-client in Cashtab
Cashtab implemented backend switching before it was integrated in the library. Deprecate Cashtab's switching mechanism and rely on the library.
Note there is an expected behavior change in Cashtab from this implementation. Previously, Cashtab would cycle through available chronik instances perpetually until one connected. Now, once all instances fail, the app will not reconnect unless it is refreshed.