feat(seo): optimise seo, metadata and sharing

## Description

Implement complete, modern SEO coverage across all pages of the Next.js 15 app — including metadata, structured data, sitemap and robots generators, Open Graph, and social previews, closely following this [guide](https://medium.com/@thomasaugot/the-complete-guide-to-seo-optimization-in-next-js-15-1bdb118cffd7)

## Expected Changes

- Leverage the Next.js **Metadata API** and `generateMetadata` to set up global (layout.tsx) and dynamic route-level metadata with OpenGraph and Twitter sections for sharing

- Dynamic robots.txt and sitemap generation (**note:** do not use `next-sitemap`, refer to the mentioned guide instead )

- Add structured data (schema markup) with `JSON-LD`

- Optimise sections hampering SEO and speed including lazy load optimisations, more SSR, deferring non critical scripts etc.

## References

- https://medium.com/@thomasaugot/the-complete-guide-to-seo-optimization-in-next-js-15-1bdb118cffd7