Add game cancellation feature with cascade deletion and host-only access control

Implemented DELETE `/games/{game_id}` endpoint allowing hosts to cancel games with cascade deletion of sheet states, chips, and memberships. Added `cancelGame` handler in App.jsx to delete game, refresh game list, reset state, and show confirmation snack. Added cancel button to GamePickerCard lobby section (pre-start) and started section (non-finished games) with bilingual confirmation dialog, loading state, and error handling.

frontend/src/components/GamePickerCard.jsx

@@ -17,11 +17,13 @@ export default function GamePickerCard({
   winnerName = "",
   chipCount = 0,
   onStartGame,
+  onCancelGame,
 }) {
   const { language, t } = useLanguage();
   const cur = games.find((x) => x.id === gameId);
   const [starting, setStarting] = useState(false);
   const [startError, setStartError] = useState("");
+  const [cancelling, setCancelling] = useState(false);
 
   const renderMemberName = (m) => {
     const base = ((m.display_name || "").trim() || (m.email || "").trim() || "—");
@@ -67,6 +69,21 @@ export default function GamePickerCard({
     }
   };
 
+  const handleCancelGame = async () => {
+    if (!onCancelGame || cancelling) return;
+    const confirmed = window.confirm(language === "en" ? "Cancel and delete this game? All game data will be removed." : "Dieses Spiel wirklich abbrechen und löschen? Alle Spieldaten werden entfernt.");
+    if (!confirmed) return;
+    setCancelling(true);
+    setStartError("");
+    try {
+      await onCancelGame();
+    } catch (e) {
+      setStartError(e?.message || (language === "en" ? "The game could not be cancelled." : "Das Spiel konnte nicht abgebrochen werden."));
+    } finally {
+      setCancelling(false);
+    }
+  };
+
   return (
     <div style={{ marginTop: 14 }}>
       <div style={styles.card}>
@@ -169,6 +186,9 @@ export default function GamePickerCard({
                 <div style={{ marginTop: 7, textAlign: "center", color: stylesTokens.textDim, fontSize: 11 }}>
                   {members.length < 2 ? t("atLeastTwoPlayers") : t("chipsOnStart")}
                 </div>
+                <button onClick={handleCancelGame} style={{ ...styles.secondaryBtn, width: "100%", marginTop: 9, color: "#ffb3b3" }} disabled={cancelling}>
+                  {cancelling ? (language === "en" ? "Cancelling …" : "Wird abgebrochen …") : (language === "en" ? "✕ Cancel game" : "✕ Spiel abbrechen")}
+                </button>
               </>
             ) : (
               <div style={{ marginTop: 11, padding: "9px 10px", borderRadius: 11, background: "rgba(233,216,166,0.07)", color: stylesTokens.textDim, fontSize: 12, textAlign: "center" }}>
@@ -179,11 +199,14 @@ export default function GamePickerCard({
             {startError && <div style={{ marginTop: 8, color: "#ffb3b3", fontSize: 12 }}>{startError}</div>}
           </div>
         ) : (
-          <div style={{ margin: "0 12px 12px", padding: "10px 12px", borderRadius: 13, border: `1px solid ${stylesTokens.panelBorder}`, background: finished ? "rgba(233,216,166,0.09)" : "rgba(124,255,182,0.07)", color: stylesTokens.textDim, fontSize: 12 }}>
-            {finished
-              ? `🏆 ${t("finished")}${winnerName ? ` · ${t("winner")}: ${winnerName}` : ""}`
-              : `✓ ${t("started")} · ${chipCount} ${t("chipsCreated")}`}
-          </div>
+          <>
+            <div style={{ margin: "0 12px 12px", padding: "10px 12px", borderRadius: 13, border: `1px solid ${stylesTokens.panelBorder}`, background: finished ? "rgba(233,216,166,0.09)" : "rgba(124,255,182,0.07)", color: stylesTokens.textDim, fontSize: 12 }}>
+              {finished
+                ? `🏆 ${t("finished")}${winnerName ? ` · ${t("winner")}: ${winnerName}` : ""}`
+                : `✓ ${t("started")} · ${chipCount} ${t("chipsCreated")}`}
+            </div>
+            {isHost && !finished && <button onClick={handleCancelGame} style={{ ...styles.secondaryBtn, margin: "0 12px 12px", width: "calc(100% - 24px)", color: "#ffb3b3" }} disabled={cancelling}>{cancelling ? (language === "en" ? "Cancelling …" : "Wird abgebrochen …") : (language === "en" ? "✕ Cancel game" : "✕ Spiel abbrechen")}</button>}
+          </>
         )}
 
         {/* Spieler */}