Enhance lesson editor: prev/next nav + richer toolbar

- page.tsx: fetch sibling lessons, pass prevLesson/nextLesson props
- LessonEditor: ChevronLeft/Right nav buttons with lesson title tooltip
- Toolbar: added Underline, Strikethrough, inline Code, H1, Horizontal rule,
  Link dialog (prompt), labeled buttons for better discoverability
- Install @tiptap/extension-underline

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -26,6 +26,7 @@
     "@tiptap/extension-image": "^3.22.2",
     "@tiptap/extension-link": "^3.22.2",
     "@tiptap/extension-placeholder": "^3.22.2",
+    "@tiptap/extension-underline": "^3.22.2",
     "@tiptap/pm": "^3.22.2",
     "@tiptap/react": "^3.22.2",
     "@tiptap/starter-kit": "^3.22.2",