feat: add TOC sidebar for README and Markdown files

- Add floating TOC panel that displays table of contents for README and markdown files
- Implement toggle button to show/hide TOC panel
- Add localStorage persistence for TOC panel visibility state
- Support both README in repo root and individual markdown file views
- Add responsive design for mobile screens

web_src/css/repo/home.css

@@ -105,3 +105,92 @@
   padding: 0 0.5em; /* make the UI look better for narrow (mobile) view */
   text-decoration: none;
 }
+
+/* File view with TOC sidebar */
+.file-view-container {
+  position: relative;
+}
+
+.file-view-container .file-view {
+  width: 100%;
+}
+
+/* TOC as a floating sidebar panel */
+.file-view-toc {
+  position: fixed;
+  top: 60px;
+  right: 0;
+  width: 280px;
+  max-height: calc(100vh - 80px);
+  overflow-y: auto;
+  padding: 1rem;
+  background: var(--color-body);
+  border: 1px solid var(--color-secondary);
+  border-right: none;
+  border-radius: var(--border-radius) 0 0 var(--border-radius);
+  box-shadow: -4px 0 12px rgba(0, 0, 0, 0.15);
+  z-index: 100;
+  transform: translateX(0);
+  transition: transform 0.25s ease-in-out;
+}
+
+/* Hidden state - using custom class to avoid Tailwind conflicts */
+.file-view-toc.toc-panel-hidden {
+  transform: translateX(100%);
+}
+
+.file-view-toc details {
+  font-size: 13px;
+}
+
+.file-view-toc summary {
+  font-weight: 600;
+  cursor: pointer;
+  padding: 8px 0;
+  color: var(--color-text);
+  border-bottom: 1px solid var(--color-secondary);
+  margin-bottom: 8px;
+}
+
+.file-view-toc ul {
+  margin: 0;
+  list-style: none;
+  padding: 5px 0 5px 1em;
+}
+
+.file-view-toc ul ul {
+  border-left: 1px dashed var(--color-secondary);
+}
+
+.file-view-toc a {
+  display: block;
+  padding: 6px 10px;
+  color: var(--color-text);
+  text-decoration: none;
+  font-size: 13px;
+  line-height: 1.4;
+  border-radius: var(--border-radius);
+}
+
+.file-view-toc a:hover {
+  color: var(--color-primary);
+  background: var(--color-hover);
+}
+
+/* TOC toggle button active state - when TOC is visible */
+#toggle-toc-btn.active {
+  color: var(--color-primary);
+  background: var(--color-hover);
+}
+
+/* Hide TOC sidebar on small screens */
+@media (max-width: 768px) {
+  .file-view-toc {
+    width: 100%;
+    max-width: 300px;
+  }
+
+  #toggle-toc-btn {
+    display: none;
+  }
+}

web_src/js/features/file-view.ts

@@ -59,9 +59,47 @@ async function renderRawFileToContainer(container: HTMLElement, rawFileLink: str
   }
 }
 
+function initTocToggle(elFileView: HTMLElement): void {
+  const toggleBtn = elFileView.querySelector('#toggle-toc-btn');
+  const tocSidebar = elFileView.querySelector('.file-view-toc');
+  if (!toggleBtn || !tocSidebar) return;
+
+  // Restore saved state from localStorage (default to hidden)
+  const savedState = localStorage.getItem('file-view-toc-visible');
+  const isVisible = savedState === 'true'; // default to hidden
+
+  // Apply initial state
+  if (isVisible) {
+    tocSidebar.classList.remove('toc-panel-hidden');
+    toggleBtn.classList.add('active');
+  } else {
+    tocSidebar.classList.add('toc-panel-hidden');
+    toggleBtn.classList.remove('active');
+  }
+
+  toggleBtn.addEventListener('click', () => {
+    const isCurrentlyVisible = !tocSidebar.classList.contains('toc-panel-hidden');
+    if (isCurrentlyVisible) {
+      // Hide TOC
+      tocSidebar.classList.add('toc-panel-hidden');
+      toggleBtn.classList.remove('active');
+      localStorage.setItem('file-view-toc-visible', 'false');
+    } else {
+      // Show TOC
+      tocSidebar.classList.remove('toc-panel-hidden');
+      toggleBtn.classList.add('active');
+      localStorage.setItem('file-view-toc-visible', 'true');
+    }
+  });
+}
+
 export function initRepoFileView(): void {
   registerGlobalInitFunc('initRepoFileView', async (elFileView: HTMLElement) => {
     initPluginsOnce();
+
+    // Initialize TOC toggle functionality
+    initTocToggle(elFileView);
+
     const rawFileLink = elFileView.getAttribute('data-raw-file-link')!;
     const mimeType = elFileView.getAttribute('data-mime-type') || ''; // not used yet
     // TODO: we should also provide the prefetched file head bytes to let the plugin decide whether to render or not