Storyline: unfoldable section list per chapter in the sidebar

Each chapter in the right-hand menu gets a caret that unfolds its
numbered sections (e.g. the prologue's 28 scenes); clicking one jumps
straight to it. The chapter you're reading starts unfolded, other
chapters' sections link across (/storyline/overview#section-…), and the
section at the top of the pane is highlighted as you scroll.

- ChapterOutline service parses each rendered chapter for its
  <h2 class="storyline-section"> headings, adds anchor ids and returns the
  outline; the controller renders all chapters once to feed the sidebar.
- storyline-toc Stimulus controller: fold/unfold, in-pane jump, scroll-spy
  (works in the split view and in the small-screen window-scroll fallback).
- Tests for the outline parser.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

assets/styles/app.css

@@ -260,6 +260,49 @@ table.dataTable {
     color: var(--bs-secondary-color);
 }
 
+/* Unfoldable per-chapter section lists */
+.storyline-toc-row { display: flex; align-items: flex-start; gap: 0.15rem; }
+.storyline-toc-row .storyline-toc-link { flex: 1 1 auto; min-width: 0; }
+.storyline-toc-fold {
+    flex: 0 0 1.4rem;
+    height: 1.9rem;
+    padding: 0;
+    border: 0;
+    background: none;
+    color: var(--bs-secondary-color);
+    cursor: pointer;
+    transition: transform 0.15s ease;
+}
+button.storyline-toc-fold:hover { color: var(--bs-body-color); }
+.storyline-toc-item.is-open > .storyline-toc-row > button.storyline-toc-fold { transform: rotate(90deg); }
+.storyline-toc-sections {
+    display: none;
+    margin: 0.1rem 0 0.4rem 1.4rem;
+    padding: 0;
+    max-height: 45vh;
+    overflow-y: auto;
+    scrollbar-width: none;
+    font-size: 0.875rem;
+}
+.storyline-toc-sections::-webkit-scrollbar { display: none; }
+.storyline-toc-item.is-open > .storyline-toc-sections { display: block; }
+.storyline-toc-sub {
+    display: flex;
+    gap: 0.5rem;
+    padding: 0.2rem 0.5rem;
+    border-left: 2px solid var(--bs-border-color);
+    text-decoration: none;
+    color: var(--bs-secondary-color);
+}
+.storyline-toc-sub:hover { color: var(--bs-body-color); background: var(--bs-tertiary-bg); }
+.storyline-toc-sub.is-current {
+    color: var(--bs-body-color);
+    border-left-color: var(--bs-primary);
+    font-weight: 600;
+}
+.storyline-toc-sub .storyline-toc-num { flex: 0 0 1.4rem; }
+.storyline-section { scroll-margin-top: 0.75rem; }
+
 /* --- Storyline: the document itself --------------------------------- */
 .storyline-doc {
     max-width: 46rem;