feat(documents): per-item Sleva (% discount) on offers + invoices

Adds an optional per-line-item percentage discount ("Sleva") to offers and
issued invoices, on both the detail-page editor and the PDF.

- DB: discount Decimal(5,2) DEFAULT 0 on quotation_items + invoice_items
  (migration 20260613195833_add_item_discount).
- Totals/VAT: new shared lineNet() (qty × price × (1 − discount/100)); offers'
  NET total and invoices' subtotal AND VAT now compute on the discounted net
  (every getBase/enrichQuotation/stats path). Backward compatible: discount 0
  is identical to before.
- Schemas: discount = numberInRange(0,100), default 0; persisted on
  create/update (+ offer duplicate).
- Detail editors: editable "Sleva %" column — offers via DocumentItemsEditor's
  new opt-in showDiscount prop (issued orders unaffected); invoices in their own
  editor. Spinners hidden + 7rem column so "100" is fully visible. Read-only
  invoice view shows the column when any line has a discount.
- PDFs (offers + invoices): conditional "Sleva"/"Discount" column rendered only
  when an item has a discount; line/total/VAT always use the discounted net.

Tests: +16 (money, totals incl. VAT-on-discounted-net, schema range, PDF
column present/absent). Full suite 665 pass, tsc -b clean, lint 0. Editor
verified live via Playwright. Design spec in docs/superpowers/specs/.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

prisma/migrations/20260613195833_add_item_discount/migration.sql

@@ -0,0 +1,6 @@
+-- AlterTable
+ALTER TABLE `invoice_items` ADD COLUMN `discount` DECIMAL(5, 2) NULL DEFAULT 0.00;
+
+-- AlterTable
+ALTER TABLE `quotation_items` ADD COLUMN `discount` DECIMAL(5, 2) NULL DEFAULT 0.00;
+

prisma/schema.prisma

@@ -208,6 +208,7 @@ model invoice_items {
   quantity         Decimal? @default(1.000) @db.Decimal(12, 3)
   unit             String?  @db.VarChar(20)
   unit_price       Decimal? @default(0.00) @db.Decimal(12, 2)
+  discount         Decimal? @default(0.00) @db.Decimal(5, 2)
   vat_rate         Decimal? @default(21.00) @db.Decimal(5, 2)
   position         Int?     @default(0)
   invoices         invoices @relation(fields: [invoice_id], references: [id], onDelete: Cascade, onUpdate: NoAction, map: "invoice_items_ibfk_1")
@@ -505,6 +506,7 @@ model quotation_items {
   quantity             Decimal?   @default(1.000) @db.Decimal(12, 3)
   unit                 String?    @db.VarChar(20)
   unit_price           Decimal?   @default(0.00) @db.Decimal(12, 2)
+  discount             Decimal?   @default(0.00) @db.Decimal(5, 2)
   is_included_in_total Boolean?   @default(true)
   modified_at          DateTime?  @db.DateTime(0)
   quotations           quotations @relation(fields: [quotation_id], references: [id], onDelete: Cascade, onUpdate: NoAction, map: "quotation_items_ibfk_1")