feat: Add comprehensive SEO improvements

- Enhanced metadata with Open Graph and Twitter Card tags
- Added robots.ts for search engine crawling directives
- Added sitemap.ts for site structure
- Improved image alt text for better accessibility and SEO
- Added JSON-LD structured data (Schema.org SoftwareApplication)
- Configured proper meta tags including keywords, authors, and robots settings
- Set metadataBase for proper URL resolution

components/file-preview-list.tsx

@@ -79,7 +79,7 @@ export function FilePreviewList({ files, onRemoveFile }: FilePreviewListProps) {
                     <div className="relative w-auto h-auto max-w-[90vw] max-h-[90vh]">
                         <Image
                             src={selectedImage}
-                            alt="Preview"
+                            alt="Full size preview of uploaded diagram or image"
                             width={1200}
                             height={900}
                             className="object-contain max-w-full max-h-[90vh] w-auto h-auto"