PERF-001: Core Web Vitals optimization — LCP, CLS, and FID improvements

## Overview
Audit and improve TechMart's Core Web Vitals scores to meet Google's 'Good' threshold. Focus on Largest Contentful Paint (LCP) and Cumulative Layout Shift (CLS).

## Acceptance Criteria
- [ ] LCP < 2.5s on mobile (throttled 4G) measured via Lighthouse
- [ ] CLS < 0.1 across all main pages
- [ ] Images have explicit width/height to prevent layout shift
- [ ] Above-the-fold content renders without render-blocking resources
- [ ] Lighthouse performance score ≥ 90 on the catalog page

## Depends On
- #36 (PDP-001 — product detail page: largest page to optimize)
- #48 (SEARCH-001 — search results page also needs optimization)

Review dependent issues #36 and #48 first to identify the product-detail and search pages involved, then run Lighthouse on the catalog page using mobile throttled 4G. Done means LCP is below 2.5s, CLS below 0.1 across main pages, images have explicit dimensions, above-the-fold content avoids render-blocking resources, and the catalog reaches a performance score of at least 90.

Written by the indexing model from the issue text.