Add optimized image for social media sharing

Update meta tags in index.html and server/static.ts to use a new, optimized 1200x630 JPG image for social media sharing, including a fallback for articles without custom images.

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server/static.ts

@@ -49,12 +49,12 @@ export function serveStatic(app: Express) {
             `<meta property="og:description" content="${escapeHtml(article.excerpt)}" />`,
             `<meta property="og:type" content="article" />`,
             `<meta property="og:url" content="${escapeHtml(articleUrl)}" />`,
-            imageUrl ? `<meta property="og:image" content="${escapeHtml(imageUrl)}" />` : "",
+            `<meta property="og:image" content="${escapeHtml(imageUrl || `${baseUrl}/og-image.jpg`)}" />`,
             `<meta property="og:site_name" content="Folx Music Television" />`,
             `<meta name="twitter:card" content="summary_large_image" />`,
             `<meta name="twitter:title" content="${escapeHtml(article.title)}" />`,
             `<meta name="twitter:description" content="${escapeHtml(article.excerpt)}" />`,
-            imageUrl ? `<meta name="twitter:image" content="${escapeHtml(imageUrl)}" />` : "",
+            `<meta name="twitter:image" content="${escapeHtml(imageUrl || `${baseUrl}/og-image.jpg`)}" />`,
             `<meta name="description" content="${escapeHtml(article.excerpt)}" />`,
             `<title>${escapeHtml(article.title)} - Volksmusik & Schlager | Folx Music Television</title>`,
           ].filter(Boolean).join("\n    ");