feat: add Markdown learning module with 8 beginner lessons

- Add markdown-basics module with lessons for headings, text formatting,
  lists, links, and inline code
- Integrate markdown section with blue color theme (#5b8dd9)
- Add markdown mode support in CodeEditor and LessonEngine
- Add markdown preview rendering using marked library
- Add section overview page with educational content
- Add markdown reference page with syntax guide
- Add i18n translations for 6 languages (EN, DE, PL, ES, AR, UK)
- Update router to recognize #markdown as section route
- Add all section-specific CSS styles for markdown theme

src/helpers/router.js

@@ -8,6 +8,7 @@
  * - #css           -> CSS section landing
  * - #html          -> HTML section landing
  * - #tailwind      -> Tailwind section landing
+ * - #markdown      -> Markdown section landing
  * - #reference/css -> CSS cheatsheet
  * - #module/index  -> Lesson (e.g., #flexbox/2)
  */
@@ -26,7 +27,7 @@ export const RouteType = {
 /**
  * Valid section IDs
  */
-const SECTIONS = ["css", "html", "tailwind"];
+const SECTIONS = ["css", "html", "tailwind", "markdown"];
 
 /**
  * Valid language codes for URL-based switching