Replace proportional split with a fill-first loop that allocates
min(remaining, deficit) to each matched month in user-supplied order,
where deficit = expected - already_paid. Prior transactions' contributions
are now properly accounted for, so a second payment on overlapping months
fills only what's still owed instead of splitting proportionally by total
expected. Surplus after all deficits are covered goes to the credit bucket.
Fixes: Matyáš Thér 200+550 showing 566/183 instead of 500/250.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
