fix: shift closing total reflects money collected, not invoiced

[NotAbdelrahmanelsayed]

Problem

When closing a POS shift, the closing total counts the full grand total of every invoice linked to the shift — even pure Pay-on-Account credit sales where no money was received. The cash Payment Reconciliation is built only from real payment rows, so it is already correct — but Net Sales (grand_total) disagrees with it within a single shift:

Event	Cash reconciliation	Net Sales (grand_total)
Credit sale created (no money)	+0 (correct)	+full amount (bug)
Old due collected today	+amount (correct)	+0

Root cause: pos_closing_shift.py › _process_invoice() added base_grand_total (the full invoice value) into grand_total/net_total/sales_total and the per-row amount, ignoring how much was actually paid.

Fix

For non-return invoices, the money summaries and the per-row grand_total now use the amount actually collected (base_paid_amount):

• pure credit sale → contributes 0
• partial sale → contributes only its down-payment
• net_total scaled by paid_ratio = base_paid / base_grand_total

The full invoice value is preserved in transaction_amount; display-only invoice_total and outstanding_amount are added for the dialog and stripped before the child-table set (Sales Invoice Reference has no such columns). Quantities, tax accrual and the returns branch are unchanged. With this change Net Sales == sum of per-invoice collected == reconciliation cash delta.

The Close Shift dialog shows an On Account / Partially Paid badge and an Unpaid: {amount} sub-line on rows collected for less than their invoice value (desktop + mobile).

Edge case: if a credit sale is created and its due is collected within the same shift, the down-payment shows in Net Sales and the later dues Payment Entry shows in reconciliation — both are real takings, reported in their respective sections.

Tests

Adds unit tests for _process_invoice covering fully-paid, pure-credit, partial and credit-return cases, asserting the summary equals only the collected portions and per-row amounts sum to the header total. Adds the Arabic translation for the new Unpaid: {0} string.

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