fix: Amount dropdown

[obany]

Description of change

The amount units dropdown did not work, because some styles had been inadvertently removed.
The amount typed in the box now has more validation, as units of "i" cannot have decimal places.

Links to any relevant issues

Fixes #298

Type of change

• Bug fix (a non-breaking change which fixes an issue)

How the change has been tested

Tested on windows, with multiple units and amounts

Change checklist

Add an x to the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes