[PERF-001] Platform: Performance optimization — lazy loading & caching

## Overview
Audit and optimize frontend performance across the app. Target a Lighthouse performance score ≥ 90 on mobile.

## Acceptance Criteria
- [ ] Lazy-load product images on scroll (Intersection Observer)
- [ ] Product catalog data cached in `sessionStorage` to avoid re-fetching on navigation
- [ ] Search index (#16) pre-built at page load and memoized
- [ ] Review section on PDP (#4, #20) loaded on demand (only when user scrolls to it)
- [ ] JavaScript split into meaningful chunks — avoid loading all of `app.js` upfront
- [ ] CSS critical path inlined; non-critical styles loaded async
- [ ] All images have explicit width/height to prevent layout shift (CLS < 0.1)
- [ ] Lighthouse mobile scores: Performance ≥ 90, Accessibility ≥ 95, Best Practices ≥ 95, SEO ≥ 90

## Depends On
- #4 PDP-001 — image optimization is tied to PDP structure
- #16 SEARCH-001 — search index caching depends on search implementation