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>

src/utils/money.ts

@@ -0,0 +1,17 @@
+/**
+ * Net amount of a single document line after its per-item percentage discount
+ * ("Sleva"). `net = qty × unit_price × (1 − discount/100)`.
+ *
+ * The discount is tolerant of null/undefined/NaN (a missing discount = none)
+ * and clamped to 0–100 so a stray value can never make the net negative.
+ * Single source of truth for offers + invoices (totals, VAT base, PDFs).
+ */
+export function lineNet(
+  qty: number,
+  unitPrice: number,
+  discountPct: number | null | undefined,
+): number {
+  const pct = Number(discountPct);
+  const safePct = Number.isFinite(pct) ? Math.min(100, Math.max(0, pct)) : 0;
+  return qty * unitPrice * (1 - safePct / 100);
+}