sync only active tab

[backpacker69]

when you switch to Assets, chizukeki still queries explorer for new transactions

[micimize]

I don't think this is necessary, now that syncing has been optimized to not refetch transactions with #14. Seeing a transaction visible on the explorer might eventually be a good way to double check papi, and the wallet data is still relevant to assets, like if they make a transaction elsewhere with the same wallet utxos will change

[micimize]

also, the user can manually disable syncing already if they really want to