fix(ui): keep Button labels on one line (whiteSpace: nowrap)

In flex rows (e.g. the add-item/add-section buttons next to a 'Ze šablony…' select on OfferDetail/OrderConfirmationModal/InvoiceDetail), flexbox shrank a button below its text width and wrapped the label onto two lines, making the button tall. The kit Button now defaults to whiteSpace: nowrap (merged with any caller sx, still overridable), so labels stay single-line app-wide.

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

src/admin/ui/Button.tsx

@@ -5,8 +5,22 @@ import type { ElementType } from "react";
  *  Accepts `component` + any extra props the component requires
  *  (e.g. `component={RouterLink} to="/foo"` or `component="a" href="…"`).
  */
-export default function Button<C extends ElementType = "button">(
-  props: ButtonProps<C, { component?: C }>,
-) {
-  return <MuiButton variant="contained" color="primary" {...props} />;
+export default function Button<C extends ElementType = "button">({
+  sx,
+  ...props
+}: ButtonProps<C, { component?: C }>) {
+  return (
+    <MuiButton
+      variant="contained"
+      color="primary"
+      {...props}
+      // Keep labels on one line — flex layouts otherwise shrink a button and
+      // wrap its text onto two lines, making the button tall. Callers can still
+      // override via their own sx.
+      sx={[
+        { whiteSpace: "nowrap" },
+        ...(Array.isArray(sx) ? sx : sx ? [sx] : []),
+      ]}
+    />
+  );
 }