[Bug]: mobile transaction amount entry is broken wrt default placeholder text and decimal point

### What happened?

Mobile amount entry on split transaction components has a pre filled text of "0.00". Typing here used to enter digits to left of wherever we had placed the cursor, and move the decimal point around accordingly. Thus we had the 0.00 as a template but could fill it in progressively. That was odd but worked fine!

Now it just appends to the pre filled 0.00 text and lets us add extra decimal points and so forth - which is basically useless, need to clear the default text before entering the real amount.


### How can we reproduce the issue?

Use mobile, enter transaction, add/edit splits.

### Where are you hosting Actual?

Pikapods

### What browsers are you seeing the problem on?

Chrome

### Operating System

Mobile Device