Improve website SEO foundation

[danielss-dev]

What changed

• reposition the homepage around Strand as a fast, cross-platform Git client while retaining AI-agent review as the differentiator
• add canonical, social, and SoftwareApplication metadata
• pre-render the Markdown user guide to clean, crawlable URLs
• generate robots.txt and sitemap.xml
• add automated SEO validation and update the public documentation references

Why

The homepage did not lead with the Git-client category, the site had no sitemap or robots file, and documentation content was only rendered after client-side JavaScript. These changes give search engines complete initial HTML, consistent canonical URLs, and clearer brand/category signals.

User and developer impact

Visitors get the same visual site and interactive demo with clearer general Git-client positioning. Documentation URLs are clean and directly crawlable, while existing ?page= links continue to redirect. Maintainers keep Markdown as the source of truth and receive build-time checks for metadata, routes, JSON-LD, and internal links.

Validation

• npm --prefix website test
• local HTTP smoke tests for homepage, docs, robots.txt, and sitemap.xml
• desktop and 390px mobile visual verification
• legacy documentation redirect verification
• interactive command-palette check
• git diff --check