[client] Fix a minimum width for the amount cell

There is a current behaviour on mobile that annoys me: when there are long transactions names when we scroll the width - and thus the text, which is truncated - of the name's cell varies if the amount's cell is bigger (ex: 30 transactions between 0.00 and 10.00 and only sometimes one transaction with 1000.00).

This fixes a minimum of 12 chars width for the amount cell.

Kind of tricky to show on a screenshot (due to scrolling) with the demo (short names etc), I hope this will be ok.