Update board placeholder styling in App.jsx

Adjusted the placeholder component with new styling properties, including changing opacity and adding a flex property. This enhances the basic layout for future integration with the game board and pieces.

frontend/src/App.jsx

@@ -393,20 +393,22 @@ export default function App() {
                 subtitle="Platzhalter – hier kommt später das Board + Figuren rein."
                 variant="panel"
               >
+                {/* 👇 DAS ist neu */}
                 <div
                   style={{
-                    height: "100%",
+                    flex: 1,
                     minHeight: 0,
                     borderRadius: 18,
                     border: `1px dashed ${stylesTokens.panelBorder}`,
-                    opacity: 0.8,
+                    opacity: 0.35,
                   }}
                 />
-                {/* Dice overlay: under the board slightly right */}
+              </PlaceholderCard>
+
+              {/* Dice overlay bleibt gleich */}
               <div className="diceOverlay">
                 <PlaceholderCard title="Würfel" variant="compact" />
               </div>
-              </PlaceholderCard>
             </div>
 
             {/* Right of board: player rail */}