feat: UI animation enhancements across all major pages

P1 - Critical UX feedback:
- Add scale-in + slide-up-in entrance animations to profile popover and notification panel
- Port SmoothedProgressBar to EcommercePage (4 generation tools: clone, detail, tryOn, productSet)
- Add result-reveal stagger animation to ecommerce result grids
- Add heart-pop spring animation to CommunityPage favorite toggle

P2 - Visual polish:
- Add scroll-entrance IntersectionObserver animations for HomePage feature sections and experience section
- Add chat-message-in entrance animation to WorkbenchPage message rows
- Fix prefers-reduced-motion accessibility in WelcomeSplash canvas (skip animation, instant entry)

P3 - CSS consolidation:
- Remove conflicting .page-motion definition from legacy-pages.css (keep translateY version from legacy-components.css)
- Consolidate skeleton-shimmer: remove opacity-pulse keyframe from primitives.css, unify with gradient sweep
- Wire up --ease-spring token for heart-pop animation
- Add :active press states (scale 0.97) to topbar buttons, brand lockup

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/hooks/useScrollEntrance.ts

@@ -0,0 +1,31 @@
+import { useEffect, useRef, useState } from "react";
+
+export function useScrollEntrance<T extends HTMLElement>(threshold = 0.15) {
+  const ref = useRef<T>(null);
+  const [isVisible, setIsVisible] = useState(false);
+
+  useEffect(() => {
+    const el = ref.current;
+    if (!el) return;
+
+    if (typeof IntersectionObserver === "undefined") {
+      setIsVisible(true);
+      return;
+    }
+
+    const observer = new IntersectionObserver(
+      ([entry]) => {
+        if (entry.isIntersecting) {
+          setIsVisible(true);
+          observer.unobserve(el);
+        }
+      },
+      { threshold },
+    );
+
+    observer.observe(el);
+    return () => observer.disconnect();
+  }, [threshold]);
+
+  return { ref, isVisible };
+}