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/impl/CodeEditor.js

@@ -8,6 +8,7 @@ import { history } from "@codemirror/commands";
 import { html } from "@codemirror/lang-html";
 import { css } from "@codemirror/lang-css";
 import { markdown } from "@codemirror/lang-markdown";
+import { javascript } from "@codemirror/lang-javascript";
 import { autocompletion } from "@codemirror/autocomplete";
 import { abbreviationTracker, expandAbbreviation } from "@emmetio/codemirror6-plugin";
 import { HighlightStyle, syntaxHighlighting } from "@codemirror/language";
@@ -181,7 +182,8 @@ export class CodeEditor {
 		const fullDoc = prefix + initialValue + suffix;
 
 		// Get language extension based on mode
-		const langExtension = this.mode === "html" ? html() : this.mode === "markdown" ? markdown() : css();
+		const langExtension =
+			this.mode === "html" ? html() : this.mode === "javascript" ? javascript() : this.mode === "markdown" ? markdown() : css();
 
 		// Create read-only zones decorations
 		const readOnlyMark = Decoration.mark({ class: "cm-readonly-zone" });