Consolidate YAML front-matter reading into one helper

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…reatures-2

Revert "Revert "Generate creatures pt. 2""

… a11y label, :active border

- GlobalNavItem is a link|group discriminated union (href required for link
  entries), so a direct-link entry can no longer render a dead <a> (#8).
- NavLogo aria-label falls back to 'Home' when brandName is omitted (#9).
- buildSupportLinks is typed UserType, not string; UserType is re-exported
  from the header package (#10).
- CreateMenu/HelpButton/HamburgerMenu read HEADER_BREAKPOINTS instead of
  hardcoded 1200/1060/1061; desktopFull is now used (#11).
- The dropdown panel surface lives once as MENU_SURFACE, shared by the menus,
  the create picker, and the hamburger popover (#12).
- New project + Account re-pin their white border in the :active block; the
  theme's Button :active rule otherwise recolors it to transparent (#16),
  guarded by a press-and-hold assertion in the story.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>