fix: StatsRing NaN with no data, Toast showing repeatedly, timer stability

- StatsRing: compute all without || 1 fallback, guard segments/legend behind hasData, use seg.count in legend to avoid NaN
- useRankChange: skip toast if already shown this session via sessionStorage
- Toast: use ref for onDismiss to prevent timer reset on every render

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

frontend/src/components/StatsRing.tsx

@@ -10,13 +10,14 @@ interface Props {
 export default function StatsRing({ exact, tendency, wrong, total }: Props) {
   const radius = 55;
   const circumference = 2 * Math.PI * radius;
-  const all = exact + tendency + wrong || 1;
+  const all = exact + tendency + wrong;
+  const hasData = all > 0;
 
-  const segments = [
-    { value: exact / all, color: 'var(--gold)', label: 'Exakt' },
-    { value: tendency / all, color: 'var(--success)', label: 'Tendenz' },
-    { value: wrong / all, color: 'var(--error)', label: 'Falsch' },
-  ];
+  const segments = hasData ? [
+    { value: exact / all, color: 'var(--gold)', label: 'Exakt', count: exact },
+    { value: tendency / all, color: 'var(--success)', label: 'Tendenz', count: tendency },
+    { value: wrong / all, color: 'var(--error)', label: 'Falsch', count: wrong },
+  ] : [];
 
   let offset = 0;
 
@@ -49,17 +50,19 @@ export default function StatsRing({ exact, tendency, wrong, total }: Props) {
         </text>
         <text x="70" y="85" textAnchor="middle"
           fill="var(--text-secondary)" fontSize="11">
-          Punkte
+          {hasData ? 'Punkte' : 'Keine Tipps'}
         </text>
       </svg>
-      <div className={styles.legend}>
-        {segments.map((seg, i) => (
-          <span key={i} className={styles.legendItem}>
-            <span className={styles.dot} style={{ background: seg.color }} />
-            {seg.label}: {Math.round(seg.value * all)}
-          </span>
-        ))}
-      </div>
+      {hasData && (
+        <div className={styles.legend}>
+          {segments.map((seg, i) => (
+            <span key={i} className={styles.legendItem}>
+              <span className={styles.dot} style={{ background: seg.color }} />
+              {seg.label}: {seg.count}
+            </span>
+          ))}
+        </div>
+      )}
     </div>
   );
 }

frontend/src/components/Toast.tsx

@@ -1,4 +1,4 @@
-import { useEffect } from 'react';
+import { useEffect, useRef } from 'react';
 import styles from './Toast.module.css';
 
 interface Props {
@@ -8,10 +8,13 @@ interface Props {
 }
 
 export default function Toast({ message, onDismiss, duration = 5000 }: Props) {
+  const onDismissRef = useRef(onDismiss);
+  onDismissRef.current = onDismiss;
+
   useEffect(() => {
-    const timer = setTimeout(onDismiss, duration);
+    const timer = setTimeout(() => onDismissRef.current(), duration);
     return () => clearTimeout(timer);
-  }, [onDismiss, duration]);
+  }, [duration]);
 
   return (
     <div className={`card ${styles.toast}`} onClick={onDismiss}>

frontend/src/hooks/useRankChange.ts

@@ -2,11 +2,15 @@ import { useState, useEffect } from 'react';
 import { api } from '../api/client';
 
 const RANK_KEY = 'tippspiel_last_rank';
+const SHOWN_KEY = 'tippspiel_rank_toast_shown';
 
 export function useRankChange() {
   const [message, setMessage] = useState<string | null>(null);
 
   useEffect(() => {
+    // Only show once per session
+    if (sessionStorage.getItem(SHOWN_KEY)) return;
+
     api.getMyStats().then(stats => {
       if (!stats.rank) return;
       const lastRank = parseInt(localStorage.getItem(RANK_KEY) || '0');
@@ -16,6 +20,7 @@ export function useRankChange() {
         } else {
           setMessage(`⬇️ Du bist auf Platz ${stats.rank} gerutscht — hol dir die Punkte zurück!`);
         }
+        sessionStorage.setItem(SHOWN_KEY, '1');
       }
       localStorage.setItem(RANK_KEY, String(stats.rank));
     }).catch(() => {});