fix(transactions): resolve bank account from cash_account_id in bulk-book and direct-book dialogs (#770)

Follow-up to #769: applies the same fetch-on-open + resolveAccount pattern to
BulkBookDialog (manual tab bank-leg pre-fill) and BookDirectlyDialog (settlement
line in buildPrefillLines). Both dialogs now fetch /api/cash-accounts when they
open, gate the pre-fill on the fetch resolving, and resolve the correct BAS
ledger account per transaction instead of always emitting '1930'.

Adds lib/cash-accounts/resolve-account.ts (cherry of the utility from #769)
since that PR is not yet merged into main.

Signed-off-by: Jonas Flodén <jonas@floden.nu>

lib/cash-accounts/resolve-account.ts

@@ -0,0 +1,31 @@
+import type { CashAccount } from '@/types'
+
+export interface ResolvedAccount {
+  account: string
+  fallback: boolean
+}
+
+/**
+ * Resolve the BAS ledger account number for a bank transaction.
+ *
+ * Resolution order:
+ * 1. cash_account_id is set → return that cash account's ledger_account.
+ * 2. Exactly one enabled account for the transaction currency → return it.
+ * 3. Give up → return '1930' with fallback=true.
+ */
+export function resolveAccount(
+  cashAccounts: CashAccount[],
+  cashAccountId: string | null,
+  currency: string,
+): ResolvedAccount {
+  if (cashAccountId) {
+    const bound = cashAccounts.find((a) => a.id === cashAccountId)
+    // If an explicit ID was given but not found, skip the currency fallback and
+    // return 1930 with fallback=true — the missing link is a data integrity signal.
+    if (bound) return { account: bound.ledger_account, fallback: false }
+    return { account: '1930', fallback: true }
+  }
+  const sameCurrency = cashAccounts.filter((a) => a.enabled && a.currency === currency)
+  if (sameCurrency.length === 1) return { account: sameCurrency[0].ledger_account, fallback: false }
+  return { account: '1930', fallback: true }
+}