fix: fix AmountInput bugs

[emccorson]

• Don't overwrite user's input string
• Clear error message when bad input is cleared
• Throw error on too many zeroes after decimal point
• Make re-delegate clear button clear input instead of setting it to 0

Fixes #905.

[github-actions[bot]]

🚀 Deployed on https://pull-request-908--wallet-development-heliax-dev.netlify.app

[pedrorezende]

LGTM! Thanks for fixing these errors, they were really annoying!