feat: redesign homepage layout and fix invite links

- Homepage: Grid-Layout mit 3 Karten (Spiel erstellen, KI, Freunde/Beitreten)
  statt vertikalem Stack mit Dividers
- CreateGame: kompakteres Styling, auf Deutsch, in Primary-Card integriert
- Einladungslink: APP_URL statt hardcoded ches.su in GamePage und CopyLink
- JoinGame: URL-Parsing für beliebige Domains (nicht nur ches.su)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/components/game/GamePage.tsx

@@ -22,7 +22,7 @@ import { Chess } from "chess.js";
 import type { ClearPremoves } from "react-chessboard";
 import { Chessboard } from "react-chessboard";
 
-import { API_URL } from "@/config";
+import { API_URL, APP_URL } from "@/config";
 import { io } from "socket.io-client";
 
 import { lobbyReducer, squareReducer } from "./reducers";
@@ -408,12 +408,8 @@ export default function GamePage({ initialLobby }: { initialLobby: Game }) {
   }
 
   function copyInvite() {
-    const text = `https://ches.su/${lobby.endReason ? `archive/${lobby.id}` : initialLobby.code}`;
-    if ("clipboard" in navigator) {
-      navigator.clipboard.writeText(text);
-    } else {
-      document.execCommand("copy", true, text);
-    }
+    const text = `${APP_URL}/${lobby.endReason ? `archive/${lobby.id}` : initialLobby.code}`;
+    navigator.clipboard.writeText(text).catch(() => {});
     setCopiedLink(true);
     setTimeout(() => {
       setCopiedLink(false);
@@ -586,7 +582,7 @@ export default function GamePage({ initialLobby }: { initialLobby: Game }) {
                   onClick={copyInvite}
                 >
                   <IconCopy size={16} />
-                  ches.su/{lobby.endReason ? `archive/${lobby.id}` : initialLobby.code}
+                  {APP_URL.replace(/^https?:\/\//, "")}/{lobby.endReason ? `archive/${lobby.id}` : initialLobby.code}
                 </label>
                 <div tabIndex={0} className="dropdown-content badge badge-neutral text-xs shadow">
                   copied to clipboard