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/admin/pages/OfferDetail.tsx

@@ -93,6 +93,7 @@ interface OfferItemPayload {
   quantity: number;
   unit: string;
   unit_price: number;
+  discount: number;
   is_included_in_total: boolean;
   position: number;
 }
@@ -303,6 +304,7 @@ export default function OfferDetail() {
             quantity: numberOr(it.quantity, 1),
             unit: it.unit || "",
             unit_price: Number(it.unit_price) || 0,
+            discount: Number(it.discount) || 0,
             is_included_in_total: it.is_included_in_total !== false,
           }))
         : [emptyDocumentItem()];
@@ -433,6 +435,7 @@ export default function OfferDetail() {
           quantity: Number(item.quantity) || 0,
           unit: item.unit,
           unit_price: Number(item.unit_price) || 0,
+          discount: Number(item.discount) || 0,
           is_included_in_total: item.is_included_in_total !== false,
           position: i,
         })),
@@ -936,6 +939,7 @@ export default function OfferDetail() {
           readOnly={readOnly}
           error={errors.items}
           showIncludedInTotal
+          showDiscount
           itemDescriptionMaxLength={8000}
           templatesSlot={
             itemTemplates && itemTemplates.length > 0 ? (
@@ -957,6 +961,7 @@ export default function OfferDetail() {
                         quantity: 1,
                         unit: "ks",
                         unit_price: template.default_price || 0,
+                        discount: 0,
                         is_included_in_total: true,
                       },
                     ]);