Fix invoice tax rounding

When a company calculates taxes with the global rounding method, the tax amounts shown on an invoice can differ slightly from the amounts recalculated line by line. This module recalculates those tax amounts and replaces them with the recalculated value.

• It only acts when the company setting for tax calculation rounding method is set to global rounding.
• Taxes whose price is included in the price are left untouched.
• If the difference between the original and recalculated amount is bigger than the allowed maximum, Odoo stops the process and shows a validation error instead of silently changing the amount.

A new field, 'Tax calculation max diff amount for rounding method', is added to the company settings and to the accounting settings screen. It appears inside the rounding method section and is only visible when the global rounding method is selected. The default value is 0, meaning any difference blocks the invoice until you raise the limit.

The module touches the invoice model account.invoice, the company model res.company and the settings model res.config.settings. It changes how tax values are computed on invoices, so it affects the amounts used in accounting entries generated from invoices.