Allow manual product cost entry in non-base currency on quotes

Summary:

When adding a product manually to a quote (without a linked distributor), the cost field only accepts input in the account's base currency. For Arc Systems, that is GBP, but they are quoting in EUR for European customers. This means the cost on the quote does not reflect the actual purchase price, making margin and P&L calculations unreliable.

Current behavior:

• Sales price on a quote correctly reflects the quoting currency (EUR)

• For distributor-linked products, cost pulls from the distributor in whatever currency they charge

• For manually added products (no distributor), cost falls back to the product-level price, which defaults to the base currency (GBP)

• There is no way to enter or override the cost in a different currency directly on the quote

Desired behavior:

• When adding a manual line item to a quote, allow the user to enter the cost in the quoting currency (e.g. EUR), not just the base currency

• Optionally: allow a "custom distributor" cost entry in a non-base currency