Unify invoices and quotes UI with discount, tax, and credit notes.

Match list/form mockups on desktop and mobile, wire real APIs for billing fields, and merge proforma into the invoices section.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/prisma/migrations/20260905160000_billing_discount_tax_kind/migration.sql

@@ -0,0 +1,9 @@
+-- AlterTable Quote
+ALTER TABLE "Quote" ADD COLUMN IF NOT EXISTS "discountAmount" DECIMAL(18,2) NOT NULL DEFAULT 0;
+ALTER TABLE "Quote" ADD COLUMN IF NOT EXISTS "taxPercent" DECIMAL(8,2) NOT NULL DEFAULT 0;
+ALTER TABLE "Quote" ADD COLUMN IF NOT EXISTS "issuedAt" TIMESTAMP(3);
+
+-- AlterTable Invoice
+ALTER TABLE "Invoice" ADD COLUMN IF NOT EXISTS "kind" TEXT NOT NULL DEFAULT 'invoice';
+ALTER TABLE "Invoice" ADD COLUMN IF NOT EXISTS "discountAmount" DECIMAL(18,2) NOT NULL DEFAULT 0;
+ALTER TABLE "Invoice" ADD COLUMN IF NOT EXISTS "taxPercent" DECIMAL(8,2) NOT NULL DEFAULT 0;