fix: rebase POS sales invoice GL entries on ERPNext 16

[my-dev-jour]

Summary

• rebase POSNext's Sales Invoice POS GL override on the ERPNext 16 core make_pos_gl_entries flow
• restore ERPNext 16's against_voucher guard and transaction-currency fields
• keep the wallet-specific party_type/party behavior on payment-mode accounts

Why

POSNext's override had drifted from ERPNext 16 core and could produce incorrect paid/outstanding behavior on submitted POS invoices.

Testing

• python3 -m py_compile pos_next/overrides/sales_invoice.py
• local verification was limited to py_compile in this workspace

Refs #191

[my-dev-jour]

Staging validation on ERPNext 16.11.0 showed the immediate root cause for the unpaid submitted invoice path was payment rows being overwritten during draft creation in pos_next/api/invoices.py, not the GL override alone.

The confirmed fix is now in PR #215, which preserves cashier-entered payment rows across set_missing_values() and was validated on staging with a real POS flow.

Closing or deprioritizing this PR in favor of #215 would avoid parallel review on the wrong root-cause path.