fix: Balance now sums past-month (paid - expected) directly, ignoring current/future months

The previous calculation derived balance from total_balance (which includes
current/future-month activity and out-of-window credits) plus a one-sided
debt-only adjustment. Current-month surplus leaked through, making the balance
appear less negative than actual past-month debt (e.g. Mauric Daniel -1250 vs
correct -1750). Pay-All is now max(0, -balance) so the two values share a
single source and cannot disagree.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## 2026-05-03 20:37 CEST — Fix Balance column to correctly reflect past-month debt
+
+- Balance (and Pay-All) are now computed as `sum(paid − expected)` over past months only, iterating directly over the ledger entries from `reconcile()`.
+- Previously the balance used `total_balance` (which includes current/future-month activity and out-of-window credits) plus a one-sided current-month debt adjustment. Current-month *surplus* leaked through, making the balance appear less negative than the actual past-month debt.
+- Pay-All is now `max(0, −balance)` so the two values are derived from a single source and can never disagree.
+- Affected: `adults_view()` and `juniors_view()` in `app.py`.
+
 ## 2026-05-03 19:26 CEST — Fee-aware allocation for multi-month payments
 
 - `reconcile()` no longer splits a multi-month payment evenly. Allocation is now per-member with two phases: greedy (if amount ≥ total expected, each month gets exactly its expected fee and overflow → credit) and proportional (otherwise distribute by each month's expected). Fixes the case where e.g. 1250 CZK covering 3 months with mixed fees (750/350/150) marked two months red.