Improve amount field checks on Send tab

[LukasKorba]

Even though there is no loss of funds possible, the UX for the amount field could be improved. There are cases when the review button is enabled with invalid amount.

Examples:

• if amount is pasted as -0.02 it's valid number but invalid Zatoshi amount. Review fails with out of bound rust error.
• if amount 0.123456789 is filled in, it's a valid number but invalid format due to too many fractional digits.

Goal

Extend the checks for valid bound and valid number of fractions.