feat(subpages): add optional sections with typographic TOC

- Add SubpageSectionConfig type to schema.ts
- Update config parser to handle sections with album overrides
- Refactor SubpageGridView to render TOC + section headers
- Add base CSS for subpage-toc and subpage-section in globals.css
- Add theme-specific overrides for all 5 themes (minimal, editorial,
  classic, noir, monograph)
- Update gallery.yaml.example and docs/gallery-config.md

app/[...path]/page.tsx

@@ -227,6 +227,7 @@ export default async function PathPage({ params }: PathPageProps) {
         subtitle={result.subpage.subtitle}
         albums={albums}
         coverPlaceholders={coverPlaceholders}
+        sections={result.subpage.sections}
       />
     );
   }