fix(ui): stack detail-page header action buttons one-per-row on mobile

Detail-page headers put their action buttons in a flex row with
flexWrap:"wrap", so on a phone the 2-4 buttons (Potvrzení / Zahájit
realizaci / Smazat, etc.) wrapped into a ragged grid.

New shared `headerActionsSx` (ui/PageHeader): on xs it's a full-width
column (each button its own full-width row); from sm up it's the usual
right-aligned wrapping row — desktop unchanged. A StatusChip kept in the
same group stays its natural size (alignSelf) instead of stretching.

Applied to PageHeader's own actions slot and every detail-page header
action group: Order/Offer/Invoice (both views)/Project + the Warehouse
Receipt/Issue/Item/Inventory detail headers.

Verified in Chrome at 430px on OrderDetail: the group is flexDirection
column with all three buttons at the same x, full-width, stacked. tsc -b
--noEmit, npm run build, vitest 152/152 clean.

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

src/admin/pages/WarehouseReceiptDetail.tsx

@@ -26,6 +26,7 @@ import {
   PageHeader,
   ConfirmDialog,
   PageEnter,
+  headerActionsSx,
   type DataColumn,
 } from "../ui";
 
@@ -301,7 +302,7 @@ export default function WarehouseReceiptDetail() {
         title={r.receipt_number || "Nový doklad"}
         subtitle={r.supplier?.name}
         actions={
-          <Box sx={{ display: "flex", alignItems: "center", gap: 1 }}>
+          <Box sx={headerActionsSx}>
             <Button
               component={RouterLink}
               to="/warehouse/receipts"