fix: calculate tax on discounted amount and add consistent rounding

[darshgupta-wisdm]

Summary

• Tax ignoring discount: Tax was calculated on the full subtotal before discount. A $400 item with $400 discount and 10% tax showed $40 instead of $0. Now applies proportional tax reduction.
• Rounding mismatches: Client-side previews could show $33.3333 while DB saves $33.33. Added Math.round(...*100)/100 to all client-side discount/tax calculations.
• Quote form subtotal: Missing per-item rounding caused quote list to show 430 while edit form showed 429. Added per-item rounding to match server.
• Quote edit form: Was missing tax entirely in total calculation.
• Legacy code: Added per-item rounding to useQuoteTotals hook and seed data generator.

Files changed (9)

• lib/invoices/actions.ts — proportional tax reduction in calculateTotals + per-item rounding in preliminarySubtotal
• lib/quotes/actions.ts — proportional tax reduction in updateQuote
• invoices/new/new-invoice-form.tsx — tax on discounted subtotal + rounding
• invoices/[id]/edit/edit-invoice-form.tsx — same
• quotes/new/new-quote-form.tsx — per-item rounding + tax on discounted subtotal
• quotes/[id]/edit/edit-quote-form.tsx — same + added missing tax calculation
• lib/stores/quote-builder-store.ts — proportional tax reduction
• components/quotes/editor/hooks/useQuoteTotals.ts — per-item rounding
• packages/database/prisma/seed.ts — 2-decimal rounding instead of integer

Test plan

• Create invoice: $100 item, 50% discount, 10% tax → tax should be $5 (not $10)
• Create quote: $400 item, $400 discount, 10% tax → total should be $0
• Verify quote list total matches quote edit form total
• Verify invoice preview matches saved values after save
• Check visual builder totals update correctly with discount + tax

Fixes #56

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