Smarter tax handling: location-based rates, exemptions, breakdowns, Stripe Tax

Make tax handling location-aware and flexible. Asks include: organization-level tax-exempt setting; per-province/state tax rates (e.g. Canadian GST); auto-applying tax codes from billing address; storing default billing address on client profile so tax computes at invoice creation; separate tax breakdown at checkout; 0% tax rates with conditional invoice notes (reverse-charge); and using Stripe Tax for automated calculation when on Stripe-hosted checkout.