refactor: simplify Journal Entry client script

- Replace the JournalEntry controller class and cur_frm.cscript free
  functions with frappe.ui.form.on event blocks plus a namespaced
  erpnext.journal_entry helper object
- Drop deprecated APIs: cur_frm/script_manager, add_fetch, $.each and var
- Move the bank_account -> account fetch to fetch_from on the
  Journal Entry Account "account" field
- Keep totals/difference and company-currency conversion on the client
  (cheap, race-free); call the server only to fetch exchange rates
- get_balance now computes its own difference instead of trusting the
  client-sent value, with a regression test

erpnext/accounts/doctype/journal_entry/journal_entry.py

@@ -889,7 +889,7 @@ class JournalEntry(AccountsController):
 			msgprint(_("'Entries' cannot be empty"), raise_exception=True)
 			return
 
-		self.total_debit, self.total_credit = 0, 0
+		self.set_total_debit_credit()
 		diff = flt(self.difference, self.precision("difference"))
 		if diff:
 			self._apply_difference_to_blank_row(diff, difference_account)

erpnext/accounts/doctype/journal_entry/test_journal_entry.py

@@ -764,6 +764,29 @@ class TestJournalEntry(ERPNextTestSuite):
 		self.assertEqual(blank_row.credit_in_account_currency, 100)
 		self.assertEqual(jv.total_debit, jv.total_credit)
 
+	def test_get_balance_recomputes_difference_ignoring_client_value(self):
+		"""get_balance computes its own difference instead of trusting a stale client-sent value."""
+		jv = frappe.new_doc("Journal Entry")
+		jv.company = "_Test Company"
+		jv.posting_date = nowdate()
+		jv.append(
+			"accounts",
+			{
+				"account": "_Test Cash - _TC",
+				"debit_in_account_currency": 100,
+				"debit": 100,
+				"exchange_rate": 1,
+			},
+		)
+		jv.append("accounts", {"account": "_Test Bank - _TC", "exchange_rate": 1})
+		# a stale/incorrect value as the client might send; get_balance must not rely on it
+		jv.difference = 0
+
+		jv.get_balance()
+		self.assertEqual(jv.accounts[1].credit_in_account_currency, 100)
+		self.assertEqual(jv.total_debit, jv.total_credit)
+		self.assertEqual(jv.difference, 0)
+
 	def test_get_outstanding_invoices_builds_write_off_rows(self):
 		"""Characterize: get_outstanding_invoices adds a party row for each outstanding invoice."""
 		from erpnext.accounts.doctype.sales_invoice.test_sales_invoice import create_sales_invoice

erpnext/accounts/doctype/journal_entry_account/journal_entry_account.json

@@ -44,6 +44,7 @@
   {
    "bold": 1,
    "columns": 4,
+   "fetch_from": "bank_account.account",
    "fieldname": "account",
    "fieldtype": "Link",
    "in_global_search": 1,