fix(home): render hero title as h1 and add meta description

SEO team flagged the home page as missing a crawlable <h1> and a meta description.

- HomeHero: promote the hero title wrapper from <div> to <h1> (all 7 locales,
  shared component); add margin:0 in SCSS to keep spacing identical.
- index.mdx: add top-level description frontmatter per locale so Rspress emits
  <meta name="description"> from the localized welcome copy.