fix: write all theme.json defaults to vault theme frontmatter

Previously, only UI chrome colours and 3 editor vars were written to
theme note frontmatter. CSS vars like --lists-bullet-size, --headings-h1-font-size,
etc. remained unset, so EditorTheme.css rules had no effect.

- Expand DEFAULT_VAULT_THEME_VARS from 46 to 140 entries, covering every
  property from theme.json (editor, headings, lists, checkboxes, inline
  styles, code blocks, blockquote, table, horizontal rule, colors)
- Fix missing px suffix on editor-font-size and editor-max-width
- Add missing semantic vars: bg-card, text-tertiary
- Refactor built-in vault themes from const strings to generated functions
  with per-theme colour overrides (DRY, all themes get editor properties)
- Quote var() references in frontmatter to prevent YAML parse issues
- Add regression tests for create_vault_theme and seed_vault_themes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/src/vault/getting_started.rs

@@ -428,17 +428,17 @@ pub fn create_getting_started_vault(target_path: &str) -> Result<String, String>
         .map_err(|e| format!("Failed to create theme directory: {e}"))?;
     fs::write(
         theme_notes_dir.join("default.md"),
-        crate::theme::DEFAULT_VAULT_THEME,
+        crate::theme::default_vault_theme(),
     )
     .map_err(|e| format!("Failed to write default vault theme: {e}"))?;
     fs::write(
         theme_notes_dir.join("dark.md"),
-        crate::theme::DARK_VAULT_THEME,
+        crate::theme::dark_vault_theme(),
     )
     .map_err(|e| format!("Failed to write dark vault theme: {e}"))?;
     fs::write(
         theme_notes_dir.join("minimal.md"),
-        crate::theme::MINIMAL_VAULT_THEME,
+        crate::theme::minimal_vault_theme(),
     )
     .map_err(|e| format!("Failed to write minimal vault theme: {e}"))?;