feat: Enhance SEO, add site files, and modernize UX

[pohlipit23]

This commit implements a series of improvements to the website:

SEO Enhancements:

• Added Open Graph and Twitter Card meta tags to index.html.
• Implemented JSON-LD structured data (FoodEstablishment schema) in index.html.
• Refined existing title, meta description, and keywords for better SEO.
• Added a canonical URL link and theme-color meta tag.
• NOTE: Placeholder URLs (https://www.example.com/) are used for canonical, OG, JSON-LD, robots.txt sitemap, and sitemap.xml loc. These MUST be updated by you.

New Files:

• Created robots.txt to guide web crawlers and link to the sitemap.
• Created sitemap.xml for better search engine indexing (single page site).

Design & UX Modernization:

• Improved sticky header with a more pronounced shadow.
• Enhanced hero section text legibility with a text shadow.
• Replaced an inline style on the hero title with a class.
• Added loading="lazy" to images for better performance.
• Improved accessibility:
  • Added ARIA attributes (aria-label, aria-expanded) to the mobile menu button and updated JS to manage aria-expanded. Added aria-live to the menu.
  • Added focus rings to primary call-to-action buttons for better keyboard navigation.
• NOTE: The brand-gold color contrast (#f7c531) should be reviewed by you for accessibility compliance.

All changes are within plain HTML, CSS, and JavaScript as requested.