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/themes/editorial.css

@@ -175,3 +175,63 @@
   font-weight: 300;
   text-transform: none;
 }
+
+/* Subpage TOC: italic, cinematic – like a film chapter list */
+[data-preset='editorial'] .subpage-toc {
+  margin-bottom: 64px;
+}
+
+[data-preset='editorial'] .subpage-toc__entry+.subpage-toc__entry::before {
+  content: '/';
+  padding: 0 12px;
+  font-family: var(--font-caption);
+  font-style: italic;
+  color: var(--text-muted);
+}
+
+[data-preset='editorial'] .subpage-toc__num {
+  font-family: var(--font-caption);
+  font-style: italic;
+  font-size: 0.65rem;
+  color: var(--text-muted);
+}
+
+[data-preset='editorial'] .subpage-toc__label {
+  font-family: var(--font-caption);
+  font-size: 0.9rem;
+  font-style: italic;
+  font-weight: 400;
+  letter-spacing: 0.04em;
+}
+
+[data-preset='editorial'] .subpage-toc__link {
+  color: var(--text-secondary);
+}
+
+[data-preset='editorial'] .subpage-toc__link:hover {
+  color: var(--accent);
+}
+
+/* Section: pull-quote accent border, large cinematic title */
+[data-preset='editorial'] .subpage-section__title {
+  font-family: var(--font-serif);
+  font-size: 2rem;
+  font-weight: 300;
+  letter-spacing: 0.01em;
+  text-transform: none;
+}
+
+[data-preset='editorial'] .subpage-section__desc {
+  font-family: var(--font-caption);
+  font-style: italic;
+  font-size: 0.85rem;
+  opacity: 0.7;
+}
+
+[data-preset='editorial'] .subpage-section__rule {
+  background: none;
+  height: 0;
+  border-left: 3px solid var(--accent);
+  margin-bottom: -16px;
+  width: 40px;
+}