fix(web): audit landing page

[MstfSlm38]

Website audit fixes for SEO, accessibility, responsive behavior, i18n and asset reliability.

Changes:

• Add Open Graph, Twitter and SoftwareApplication metadata.
• Add canonical URL.
• Use repository screenshots instead of GitHub user-attachment URLs.
• Add Backup & Restore and Customization feature cards.
• Fix roadmap i18n and clarify 26Q4 macOS focus.
• Fix mobile menu state and close it after navigation.
• Add keyboard focus styles and reduced-motion support.
• Prevent scroll-reveal from hiding content when JavaScript is unavailable.
• Move website styling out of JavaScript.
• Improve small screen layout and CSS cleanup.