ui(dashboard): centre-align Hinzufuegen button on mobile

Switch the title-row flex from items-start to items-center
sm:items-start. The dashboard.title plus welcomeText sub-line
wraps to two lines on < 380 px viewports and the button used to
hug the top of that 2-line block without a baseline anchor.
items-center on mobile centres the button against the title; sm+
returns to the original top-aligned posture where the title is
one line.

src/app/page.tsx

@@ -526,7 +526,14 @@ export default function DashboardPage() {
 
   return (
     <div className="space-y-6">
-      <div className="flex items-start justify-between gap-4">
+      {/* v1.4.37 W4a item 7 — centre-align the Hinzufügen button
+          against the 2-line title block on mobile (< sm). The
+          `welcomeText` line wraps under the title at < 380 px so the
+          button used to float at the top of the row without a
+          baseline anchor; `items-center sm:items-start` keeps the
+          mobile vertical centre while preserving the original
+          top-aligned posture on sm+ (where the title is one line). */}
+      <div className="flex items-center justify-between gap-4 sm:items-start">
         <div>
           <h1 className="text-2xl font-bold tracking-tight">
             {t("dashboard.title")}