Update Split table logic to update amounts/percentages to match the original transaction amount

[garrettmknight]

Holding on #66064 to implement for both % and amounts.

In Classic, when a user splits an expense, we try to update the amounts for all splits to equal the amount of the original transaction automatically. We opted to go a simpler route with NewDot splits to start, avoiding any automatic updates, and punted adding this logic to a follow-up. This is that follow-up:

Problem: When users are editing the separate portions of a split they are typically trying to ensure all the portions add up to the original transaction amount.

Solution: Automatically change amounts and percentages when creating a split to add up to the original transaction amount to remove clicks/steps for the submitter. To do so, we'll implement our Classic logic:

Core logic:

• Default/unmodified values adjust automatically: Any split that hasn't been manually edited (i.e., still at its default value) will auto-adjust evenly so that all splits add up to 100% (for percentages) or the total amount (for amounts).
• Edited values are preserved: Once a user manually edits a percentage (or amount), that value is “locked in” and will never be auto-updated.
• Editing an initial split will preserve all Split values: We've added the ability to edit existing splits in NewDot and the individaul splits of an expense may be on different reports and in different states. To avoid complexity, when a user edits an amount or percentage on an existing split, we treat all splits as manually edited and stop auto-adjusting them.

Behavior for Adding Splits

• When a new split is added: Instead of defaulting to 0%, the system will redistribute all unedited splits automatically so they balance out to 100%.

Simple adding splits Example:

1. Submitter taps More > Split on a $10 expense
2. Two initial splits exist at 50/50 = $5/$5
3. Submitter taps 'Add split'
4. A third split is added and the amounts are updated to 33%/33%/34% = $3.33/$3.33/$3.34

Further editing example:

1. Submitter taps More > Split on a $10 expense
2. Two initial splits exist at 50/50 = $5/$5
3. Submitter edits Split A to $3.
4. Split B automatically updates to $7
5. Submitter taps 'Add split'
6. A third split is added. A remains $3, B updates to $3.50, the new split C is added as $3.50.

Error handling:

• Users can still manually override and push totals over/under 100%.
• In those cases, the same existing error message for invalid totals will be shown.

[garrettmknight]

Still holding on Split by.

[garrettmknight]

Still holding on Split by.

[garrettmknight]

Still holding on Split by, but we're getting closer.

[garrettmknight]

Split by should be done in a few weeks, then we'll get this going.