feat: add command palette, accessibility, scroll animations, and keyboard navigation

Implements COMP-139 (command palette), COMP-140 (accessibility), COMP-141 (scroll animations), COMP-145 (keyboard navigation)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-Claude)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

apps/web/src/hooks/use-scroll-animate.ts

@@ -0,0 +1,52 @@
+"use client";
+
+import { useEffect, useRef } from "react";
+
+interface UseScrollAnimateOptions {
+  threshold?: number;
+  rootMargin?: string;
+  once?: boolean;
+}
+
+export function useScrollAnimate<T extends HTMLElement = HTMLDivElement>({
+  threshold = 0.1,
+  rootMargin = "0px 0px -60px 0px",
+  once = true,
+}: UseScrollAnimateOptions = {}) {
+  const ref = useRef<T>(null);
+
+  useEffect(() => {
+    const el = ref.current;
+    if (!el) return;
+
+    const prefersReducedMotion = window.matchMedia(
+      "(prefers-reduced-motion: reduce)"
+    ).matches;
+
+    if (prefersReducedMotion) {
+      el.setAttribute("data-animate", "visible");
+      return;
+    }
+
+    const observer = new IntersectionObserver(
+      (entries) => {
+        entries.forEach((entry) => {
+          if (entry.isIntersecting) {
+            entry.target.setAttribute("data-animate", "visible");
+            if (once) {
+              observer.unobserve(entry.target);
+            }
+          } else if (!once) {
+            entry.target.setAttribute("data-animate", "hidden");
+          }
+        });
+      },
+      { threshold, rootMargin }
+    );
+
+    observer.observe(el);
+    return () => observer.disconnect();
+  }, [threshold, rootMargin, once]);
+
+  return ref;
+}