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;
+  }
+}