feat: rank change toast + streak milestone icons

Toast notification on rank change (up/down).
Streak display with milestones: 🔥 at 3, 🔥🔥 at 10, ⚡ at 20.

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

frontend/src/pages/DashboardPage.tsx

@@ -7,6 +7,14 @@ interface Props {
   devUser?: number;
 }
 
+function formatStreak(streak: number): string {
+  if (streak >= 20) return `⚡${streak}`;
+  if (streak >= 10) return `🔥🔥${streak}`;
+  if (streak >= 3) return `🔥${streak}`;
+  if (streak > 0) return String(streak);
+  return '0';
+}
+
 function formatCountdown(minutes: number): string {
   if (minutes < 60) return `in ${minutes} Min`;
   if (minutes < 60 * 24) return `in ${Math.floor(minutes / 60)}h`;
@@ -108,9 +116,7 @@ export default function DashboardPage(_props: Props) {
           <span className={styles.statLabel}>Punkte</span>
         </div>
         <div className={styles.statTile}>
-          <span className={styles.statValue}>
-            {stats.streak > 0 ? `${stats.streak} 🔥` : stats.streak}
-          </span>
+          <span className={styles.statValue}>{formatStreak(stats.streak)}</span>
           <span className={styles.statLabel}>Streak</span>
         </div>
       </div>