Apply supplier credits across projects

Problem
Today, a supplier credit in Billdr is tied to a single project. In practice, suppliers like Home Depot apply credits to the contractor's account, not to a specific job. When the contractor has unpaid invoices from that supplier on several projects, they have to split the credit by hand and allocate each part to a project before they can apply it.

Customer example
The customer has a $1,740.86 credit with Home Depot and unpaid HOME DEPOT invoices on several projects. To use the credit, they had to split it manually across two projects ahead of time.

Requested behavior

• Record a supplier credit at the supplier/account level, without assigning it to a project.

• Apply it to any open invoice from that supplier, whatever project the invoice belongs to.

• Allow partial application, so one credit can cover several invoices and the remaining balance carries over.

• Show the remaining credit balance per supplier.

• Keep job costing accurate: each project only shows the portion of the credit applied to its invoices.

Why it matters

• Billdr would match how suppliers actually issue credits (on the account, not per job).

• No more manual pre-splitting, and fewer allocation errors.

• Reconciling against supplier statements becomes cleaner.