Namadillo: Disable MASP transfers until synced

[Rigorously]

Namadillo currently allows initiating a MASP transfer while the shielded-sync is still processing. That will of course cause an error.

Namadillo should signal to the user that shielded-sync is busy and prevent the user submitting a transfer. Allowing the user to fill form input fields during syncing could be acceptable, although the balance will be outdated. Although user may switch to the MASP page to check the sync status and then lose the form input. Disabling form input may prevent user from wasting time.

Maybe shielded-sync status could be included in the current RPC/indexer sync status indicator.