fix(ui): detail-form headers no longer overflow on mobile

Two causes: theme h4 had no responsive size (desktop 2.125rem on
phones - now 1.5rem under 600px, applies to all page headlines), and
the Zpet+title header row had no flexWrap so long document titles
pushed past the viewport (issued orders, offers, both invoice views).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/admin/pages/IssuedOrderDetail.tsx

@@ -573,7 +573,16 @@ export default function IssuedOrderDetail() {
           mb: 3,
         }}
       >
-        <Box sx={{ display: "flex", alignItems: "center", gap: 2 }}>
+        {/* flexWrap: long document titles must drop below the Zpět button on
+            phones instead of overflowing the viewport. */}
+        <Box
+          sx={{
+            display: "flex",
+            alignItems: "center",
+            gap: 2,
+            flexWrap: "wrap",
+          }}
+        >
           <Button
             component={RouterLink}
             to="/orders?tab=vydane"