Add account balance anchors

internal/app/import.go

@@ -882,6 +882,7 @@ func (a *App) Sync(ctx context.Context) (State, error) {
 		}
 		s = result.State
 		a.ops.AccountSync[account.ID] = now.Format(time.RFC3339)
+		s = a.anchorAccount(ctx, s, account, to)
 	}
 	a.ops.SyncError = strings.Join(failures, "; ")
 	a.ops.SyncRetryAt = ""
@@ -897,6 +898,66 @@ func (a *App) Sync(ctx context.Context) (State, error) {
 	return a.snapshot(ctx)
 }
 
+// anchorAccount fixes a connected cash account's start balance after its first
+// successful sync: the bank's booked (CLBD) balance is captured once, verbatim,
+// with the day it was true, so a date-windowed history still yields the real
+// balance — the money from before the window is derived as the anchor less
+// every movement booked through the anchor date, and an older import later
+// corrects that derivation by itself. The balance is fetched after the
+// transactions to minimize the gap between the two reads. Banks supply booking
+// dates rather than exact times, so the anchor day is deliberately treated as
+// one completed booked state. Every failure leaves the anchor unset for the
+// next sync to retry; a missing CLBD figure is such a failure, because an
+// available or expected balance includes pending amounts that have no booked
+// fact to subtract.
+func (a *App) anchorAccount(ctx context.Context, s State, account domain.Account, today string) State {
+	if account.Investing() || account.AnchorDate != "" || account.ExternalAccountID == "" {
+		return s
+	}
+	balances, err := a.bank.Balances(ctx, account.ExternalAccountID)
+	if err != nil {
+		return s
+	}
+	var selected banking.Balance
+	anchorDate := ""
+	for _, balance := range balances {
+		if balance.Type != "CLBD" || balance.Currency != account.Currency {
+			continue
+		}
+		date := balance.ReferenceDate
+		if date == "" {
+			date = today
+		} else if _, e := time.Parse("2006-01-02", date); e != nil || date > today {
+			continue
+		}
+		if date < anchorDate {
+			continue
+		}
+		// Two different booked figures for the same account, currency and
+		// reference day are ambiguous. Do not let response order decide money.
+		if date == anchorDate && anchorDate != "" && balance.Amount != selected.Amount {
+			return s
+		}
+		selected, anchorDate = balance, date
+	}
+	if anchorDate == "" {
+		return s
+	}
+	data := domain.Clone(s.Data)
+	for i := range data.Accounts {
+		if data.Accounts[i].ID != account.ID {
+			continue
+		}
+		data.Accounts[i].AnchorBalance = selected.Amount
+		data.Accounts[i].AnchorDate = anchorDate
+		if next, e := a.commit(ctx, s.Revision, data); e == nil {
+			return next
+		}
+		return s
+	}
+	return s
+}
+
 // syncInterval is how often connected accounts synchronize on their own. Twice
 // a day halves how long a booking can sit unseen while staying inside Enable
 // Banking's documented background allowance of roughly four fetches per day per