Update locale spec: add UI design, landing page, and hreflang

- Add Section 2: Locale Picker UI Design based on javaevolved.github.io
  reference (HTML markup, CSS, JS behavior, placement rules, SEO hreflang)
- Add Section 10: Landing page (copilot-dev-days.github.io) localization
- Update scope to include the landing page repository
- Update implementation order, cross-repo rules, and quality checklist
- Renumber all sections for consistency

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: brunoborges