feat: add JavaScript learning section with starter lessons and sidebar section headers

Implementation following plan:
- S01: Foundation: schema, section config, and router
- S02: Install CodeMirror JavaScript language support
- S03: Create JavaScript lesson JSON files (variables, DOM, events)
- S04: Register JavaScript lessons in module stores
- S05: Add JavaScript validation logic
- S06: Add JavaScript mode to LessonEngine preview rendering
- S07: Add JavaScript mode to CodeEditor
- S08: Update app.js for JavaScript mode support
- S09: Update navigation HTML and CSS theming for JavaScript section
- S10: Add section grouping headers in sidebar navigation
- S11: Update and write tests

src/helpers/router.js

@@ -27,7 +27,7 @@ export const RouteType = {
 /**
  * Valid section IDs
  */
-const SECTIONS = ["css", "html", "markdown"]; // tailwind temporarily disabled
+const SECTIONS = ["css", "html", "markdown", "javascript"]; // tailwind temporarily disabled
 
 /**
  * Valid language codes for URL-based switching