feat(ui): add table of contents for chapter navigation

Added table of contents modal for documents with multiple chapters:

New Features:
- TableOfContents component with modal dialog
- List icon button in chapter navigation bar
- Click any chapter to jump directly to it
- Current chapter highlighted in the list
- Scrollable list for books with many chapters
- Clean modal UI with transitions

Benefits:
- Quick navigation for large documents with many chapters
- Better UX for series/textbooks with 10+ chapters
- No need to click Previous/Next repeatedly
- Shows all chapter titles at a glance

Technical Details:
- Uses Headless UI Dialog for accessibility
- Integrates with existing goToChapter function
- Matches existing design system and styling
- Added to HTMLViewer chapter navigation bar

src/components/HTMLViewer.tsx

@@ -5,6 +5,7 @@ import ReactMarkdown from 'react-markdown';
 import remarkGfm from 'remark-gfm';
 import { useHTML } from '@/contexts/HTMLContext';
 import { DocumentSkeleton } from '@/components/DocumentSkeleton';
+import { TableOfContents } from '@/components/TableOfContents';
 
 interface HTMLViewerProps {
   className?: string;
@@ -19,6 +20,7 @@ export function HTMLViewer({ className = '' }: HTMLViewerProps) {
     totalChapters,
     goToNextChapter,
     goToPreviousChapter,
+    goToChapter,
   } = useHTML();
   const containerRef = useRef<HTMLDivElement>(null);
 
@@ -48,13 +50,20 @@ export function HTMLViewer({ className = '' }: HTMLViewerProps) {
             Previous
           </button>
 
-          <div className="flex items-center gap-2">
+          <div className="flex items-center gap-3">
-            <span className="text-sm text-foreground font-medium">
+            <div className="flex items-center gap-2">
-              {currentChapter?.title || `Chapter ${currentChapterIndex + 1}`}
+              <span className="text-sm text-foreground font-medium">
-            </span>
+                {currentChapter?.title || `Chapter ${currentChapterIndex + 1}`}
-            <span className="text-xs text-muted">
+              </span>
-              ({currentChapterIndex + 1} of {totalChapters})
+              <span className="text-xs text-muted">
-            </span>
+                ({currentChapterIndex + 1} of {totalChapters})
+              </span>
+            </div>
+            <TableOfContents
+              chapters={chapters}
+              currentChapterIndex={currentChapterIndex}
+              onChapterSelect={goToChapter}
+            />
           </div>
 
           <button

src/components/TableOfContents.tsx

@@ -0,0 +1,122 @@
+'use client';
+
+import { Fragment, useState } from 'react';
+import { Dialog, DialogPanel, DialogTitle, Transition, TransitionChild } from '@headlessui/react';
+import { ListIcon } from '@/components/icons/Icons';
+import type { Chapter } from '@/lib/chapterDetection';
+
+interface TableOfContentsProps {
+  chapters: Chapter[];
+  currentChapterIndex: number;
+  onChapterSelect: (index: number) => void;
+}
+
+export function TableOfContents({ chapters, currentChapterIndex, onChapterSelect }: TableOfContentsProps) {
+  const [isOpen, setIsOpen] = useState(false);
+
+  const handleChapterClick = (index: number) => {
+    onChapterSelect(index);
+    setIsOpen(false);
+  };
+
+  return (
+    <>
+      {/* Table of Contents Button */}
+      <button
+        onClick={() => setIsOpen(true)}
+        className="inline-flex items-center px-3 py-1 text-sm rounded-md border border-muted bg-base text-foreground hover:bg-offbase transition-all duration-200 ease-in-out hover:scale-[1.04] hover:text-accent"
+        aria-label="Table of contents"
+        title="Table of contents"
+      >
+        <ListIcon className="w-4 h-4" />
+      </button>
+
+      {/* Table of Contents Modal */}
+      <Transition appear show={isOpen} as={Fragment}>
+        <Dialog
+          as="div"
+          role={undefined}
+          className="relative z-50"
+          onClose={() => setIsOpen(false)}
+        >
+          <TransitionChild
+            as={Fragment}
+            enter="ease-out duration-300"
+            enterFrom="opacity-0"
+            enterTo="opacity-100"
+            leave="ease-in duration-200"
+            leaveFrom="opacity-100"
+            leaveTo="opacity-0"
+          >
+            <div className="fixed inset-0 overlay-dim backdrop-blur-sm" />
+          </TransitionChild>
+
+          <div className="fixed inset-0 overflow-y-auto">
+            <div className="flex min-h-full items-center justify-center p-4 text-center">
+              <TransitionChild
+                as={Fragment}
+                enter="ease-out duration-300"
+                enterFrom="opacity-0 scale-95"
+                enterTo="opacity-100 scale-100"
+                leave="ease-in duration-200"
+                leaveFrom="opacity-100 scale-100"
+                leaveTo="opacity-0 scale-95"
+              >
+                <DialogPanel role='dialog' className="w-full max-w-2xl transform rounded-2xl bg-base p-6 text-left align-middle shadow-xl transition-all">
+                  <DialogTitle
+                    as="h3"
+                    className="text-lg font-semibold leading-6 text-foreground mb-4"
+                  >
+                    Table of Contents
+                  </DialogTitle>
+
+                  {/* Chapters List */}
+                  <div className="mt-4 max-h-[60vh] overflow-y-auto">
+                    <div className="space-y-2">
+                      {chapters.map((chapter, index) => (
+                        <button
+                          key={index}
+                          onClick={() => handleChapterClick(index)}
+                          className={`w-full text-left px-4 py-3 rounded-lg transition-all duration-200 ease-in-out
+                            ${index === currentChapterIndex
+                              ? 'bg-accent text-background font-medium'
+                              : 'bg-offbase text-foreground hover:bg-muted hover:text-accent hover:scale-[1.02]'
+                            }`}
+                        >
+                          <div className="flex items-center justify-between">
+                            <div className="flex-1">
+                              <div className="font-medium">
+                                {chapter.title}
+                              </div>
+                            </div>
+                            <div className="text-sm opacity-70 ml-2">
+                              {index + 1} / {chapters.length}
+                            </div>
+                          </div>
+                        </button>
+                      ))}
+                    </div>
+                  </div>
+
+                  {/* Close Button */}
+                  <div className="mt-6 flex justify-end">
+                    <button
+                      type="button"
+                      className="inline-flex justify-center rounded-lg bg-background px-4 py-2 text-sm
+                               font-medium text-foreground hover:bg-offbase focus:outline-none
+                               focus-visible:ring-2 focus-visible:ring-accent focus-visible:ring-offset-2
+                               transform transition-transform duration-200 ease-in-out hover:scale-[1.04] hover:text-accent"
+                      onClick={() => setIsOpen(false)}
+                    >
+                      Close
+                    </button>
+                  </div>
+                </DialogPanel>
+              </TransitionChild>
+            </div>
+          </div>
+        </Dialog>
+      </Transition>
+    </>
+  );
+}