Customer-Assigned Tax Rates

Assign a default tax rate to each customer so the correct rate is applied automatically when invoicing them, rather than entering it manually each time. Also support marking customers as tax-exempt so no tax is applied to their invoices.

Particularly relevant for US sales tax (where rates vary by county/state and customer location determines the applicable rate) and any other jurisdiction where tax is customer-specific rather than business-specific.

Complements the existing per-line-item tax entry, which handles taxable vs non-taxable items, while this entry handles the customer-level rate assignment.