feat: add vertical reading progress indicator

components/ui/ReadingProgress.tsx

@@ -0,0 +1,35 @@
+"use client";
+
+import { useEffect, useState } from "react";
+
+export function ReadingProgress() {
+  const [progress, setProgress] = useState(0);
+
+  useEffect(() => {
+    if (window.CSS?.supports("animation-timeline", "scroll()")) {
+      return;
+    }
+
+    const update = () => {
+      const article = document.querySelector("article");
+      if (!article) return;
+      const rect = article.getBoundingClientRect();
+      const total = article.scrollHeight;
+      const scrolled = Math.max(0, -rect.top + window.innerHeight * 0.3);
+      setProgress(total > 0 ? Math.min(100, (scrolled / total) * 100) : 0);
+    };
+
+    update();
+    window.addEventListener("scroll", update, { passive: true });
+    return () => window.removeEventListener("scroll", update);
+  }, []);
+
+  return (
+    <div className="reading-progress hidden lg:block">
+      <div
+        className="reading-progress-fill"
+        style={{ height: `${progress}%` }}
+      />
+    </div>
+  );
+}