Validates that all 140 CSS custom properties from the expanded
DEFAULT_VAULT_THEME_VARS are applied to the DOM when a theme is
activated, including editor, heading, list, checkbox, inline-style,
code-block, blockquote, table, and horizontal-rule properties.
Also updates mock content and handlers to use the full 140-property
frontmatter, matching the Rust backend output.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>