Volume in the invoices analysis view

This module extends the invoices analysis view (the reporting screen built on account.invoice.report) with a new measure called Volume.

• The Volume value is calculated from the product's volume, multiplied by the invoiced quantity and converted using the line's unit of measure and the product's reference unit of measure.
• For vendor bills, customer refunds and incoming receipts, the volume is counted as negative, so returns and refunds reduce the total.
• If a product has no volume defined, the value is shown as zero instead of causing an error.

The new measure appears in the same analysis view where users already review invoice lines, so they can group and filter by customer, product or period and see the total volume alongside amounts. No new menus, buttons or scheduled actions are added; the change is limited to the reporting model account.invoice.report and its Volume field.