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/components/Toast.tsx

@@ -0,0 +1,21 @@
+import { useEffect } from 'react';
+import styles from './Toast.module.css';
+
+interface Props {
+  message: string;
+  onDismiss: () => void;
+  duration?: number;
+}
+
+export default function Toast({ message, onDismiss, duration = 5000 }: Props) {
+  useEffect(() => {
+    const timer = setTimeout(onDismiss, duration);
+    return () => clearTimeout(timer);
+  }, [onDismiss, duration]);
+
+  return (
+    <div className={styles.toast} onClick={onDismiss}>
+      {message}
+    </div>
+  );
+}