Add 'available balance' display to send modal

[shrnkld]

We need to show the available balance on the send modal UI so users know what funds they have available before sending.

To do:

• [x] add availableBalance to send funds modal

[shrnkld]

QA test:

• [ ] send tx modal shows the available balance under to the amount field for any asset selected

See screenshot for example of expected behaviour:

Test builds here: https://gitlab.com/blocknetdx/xlite/-/pipelines/344896422