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/InvoiceDetail.tsx

@@ -64,6 +64,7 @@ import {
   LoadingState,
   PageEnter,
   RichTextView,
+  headerActionsSx,
 } from "../ui";
 
 const API_BASE = "/api/admin";
@@ -1114,14 +1115,7 @@ export default function InvoiceDetail() {
                 />
               </Box>
             </Box>
-            <Box
-              sx={{
-                display: "flex",
-                alignItems: "center",
-                gap: 1,
-                flexWrap: "wrap",
-              }}
-            >
+            <Box sx={headerActionsSx}>
               {hasPermission("invoices.export") && (
                 <Button
                   onClick={() => handleViewPdf(invoice.language || "cs")}
@@ -1660,14 +1654,7 @@ export default function InvoiceDetail() {
               )}
             </Box>
           </Box>
-          <Box
-            sx={{
-              display: "flex",
-              alignItems: "center",
-              gap: 1,
-              flexWrap: "wrap",
-            }}
-          >
+          <Box sx={headerActionsSx}>
             {isEdit && invoice && hasPermission("invoices.export") && (
               <Button
                 onClick={() => handleViewPdf(invoice.language || "cs")}