feat(seo): theme-color 메타 추가 및 hero 이미지 width/height로 CLS 개선(TDD)

lib/exporter/html.ts

@@ -124,7 +124,7 @@ function renderHero(section: { props: { heading: string; subheading?: string; im
           ${subheading.length > 0 ? `<p>${escapeHtml(subheading)}</p>` : ''}
         </div>
         <div class="media">
-          <img src="${escapeHtml(imageUrl)}" alt="${escapeHtml(heading)}" loading="lazy" decoding="async" />
+          <img src="${escapeHtml(imageUrl)}" alt="${escapeHtml(heading)}" loading="lazy" decoding="async" width="1200" height="675" />
         </div>
       </div>
     </section>
@@ -256,6 +256,7 @@ function buildHead(page: Page) {
     <link rel="canonical" href="/" />
     <link rel="manifest" href="/site.webmanifest" />
     <link rel="icon" href="/favicon.ico" />
+    <meta name="theme-color" content="${page.theme.primaryColor}" />
     <title>${escapeHtml(page.seo.title)}</title>
     <meta name="description" content="${escapeHtml(page.seo.description)}" />
     <meta property="og:title" content="${escapeHtml(page.seo.title)}" />