A discount you type while editing a contract no longer disappears

In a subscription contract's Edit > Product details modal, typing a discount on one line and then changing a different line's product used to silently revert your typed discount back to what it was before, with no indication anything had been undone.

What's fixed: an edit to a line that still exists in the modal now survives a product change made elsewhere in the same session. If you replace the product on the line you just edited, so there's genuinely nothing left to carry the discount to, a banner now names the product and says plainly that the edit wasn't carried over, instead of Save silently doing nothing.

Saving the modal also no longer sends a 0% discount for a line you didn't touch; only the lines you actually edited are included in the save.