Agent UI v0.4.3

Agent UI v0.4.3 fixes and polishes the localized documentation home pages so /en/ and /zh/ are proper landing pages for readers and AI clients.

Highlights

• Converts localized index pages into proper layout: home landing pages.
• Adds clearer hero actions for the specification, quickstart, standards ecosystem, and LLM context.
• Keeps the homepage concise while routing detailed concepts to dedicated documentation pages.
• Fixes the GitHub Pages logo path so repository-base deployments load the site logo correctly.
• Keeps the core Agent UI specification compatible with v0.4.2.